You have a good size tank, so that's very important to keeping the temperature gradient correct, as a lot of the time people have a small tank that is impossible to get the gradient correct in. Once you get the correct wattage of bright-white colored Basking Bulb over the Hot Side of the tank and get everything worked out, it pretty much will stay the same temperature gradient, unless there is an outside-influence on the tank, like a fan, a drop or raise in the room temperature that is significant, the tank is right near a window with the sun beating in, etc.
Think of it this way: Your goal with his lighting is to replicate natural sunlight as closely as you possibly can OVER THE HOT SIDE OF THE TANK, BECAUSE THAT IS WHERE HIS BASKING SPOT/PLATFORM IS LOCATED. You accomplish this by putting a long, strong, fluorescent UVB tube that is at least a 10% (10.0) or higher UVB tube, preferably a T5 strength, over the Hot Side of the tank, and RIGHT ALONGSIDE A BRIGHT-WHITE COLORED BASKING BULB, so that both lights are "SHARING" the top of the Hot Side of the tank (T8 tubes are okay as long as they are underneath the mesh lid and within 6" of the basking spot/platform, and replaced every 6 months, but a T5 is much better for him). So basically you want both of these lights off-center over the Hot Side of the tank, with one light closer to the back, one light closer to the front, but both closer to the center of the Hot Side...I say this because you do not want the UVB tube right up against the back or front glass, you want it closer to the center of the Hot Side of the tank.
Then you want to place his main basking spot/platform within the Hot Side of the tank and directly underneath both lights, so that your Dragon will get both lights while basking.
The wattage of the bright-white colored Basking Bulb over the Hot Side of the tank is going to be SOLELY responsible for the Basking Spot Surface Temperature and the Hot Side Ambient/Air Temperature, and will of course influence the Cool Side Ambient/Air Temperature on the opposite side of the tank, but for now, concentrate only on the Basking Spot Surface Temp and the Hot Side Ambient/Air Temp.
For a 40 Gallon Breeder Tank you usually can start out with a 100 watt bright-white colored Basking Light, and then go from there, usually a 100 watt will do it with minor adjustments. So once you get both the UVB tube and the Basking Light both set-up sharing the center of the Hot Side of the tank, and you get his basking spot/platform directly underneath both lights on the Hot Side (MAKING SURE THAT THE BASKING SPOT/PLATFORM IS WITHIN THE CORRECT DISTANCE FROM THE UVB TUBE, EITHER 6" FOR A T8 OR 11" FOR A T5), then allow the lights to stay on for about an hour so that the everything reaches temperature. Then you place your Probe from your Digital Thermometer directly on the basking spot/platform, right on the spot your dragon sits/lays while basking, AND ALLOW THE PROBE TO SIT FOR AT LEAST 20-30 MINUTES SO THAT THE PROBE WILL REACH THE ACTUAL SURFACE TEMPERATURE! This is extremely important and a step that people skip, and obviously the temperature will read as being much lower than it actually is if you don't give the Probe enough time to heat up, so make sure the temperature on the Digital Thermometer is no longer rising before you read it...
Okay, so you've now got your Basking Spot Surface Temperature...Now, if it turns out that it's WAY OFF, like more than 15-20 degrees off in either direction, then you pretty much need to just switch to a higher or lower wattage of bright-white colored basking bulb. However, lets say you get a Basking Spot Surface Temperature of 115 degrees F, and you need it to be within 105-110 degrees F and no higher than 110, as this is the highest Surface Temp that should occur anywhere inside a Dragon Tank. So in this example you need to drop the Basking Spot Surface Temperature about 5 or so degrees. ALWAYS REMEMBER THAT MOVING THE BASKING SPOT/PLATFORM AWAY, OR MAKING IT SHORTER IS NOT AN OPTION, AS IT MUST REMAIN WITHIN THE CORRECT DISTANCE FROM THE UVB TUBE...But you can move the Basking Bulb up, and that's what you have to do. So move the Basking Bulb up away a few inches from the basking spot/platform, let it sit for another 30 minutes or so, and then take the Basking Spot Surface Temperature again, allowing the Probe to sit for 20-30 minutes, or until the temp stops rising. This is how you make minor adjustments to the Basking Spot Surface Temperature. If you son't already have an adjustable stand for the Basking Bulb, i suggest you buy one, they attach to the underside of the tank and you suspend the Basking Bulb inside it's fixture from them above the tank, and they allow you to raise/lower the Basking Bulb. They cost like $20 at any Petco or PetSmart, and well worth the money.
Always start with the Basking Spot Surface Temperature. Once you get it within the correct range, then you next want to measure the Hot Side Ambient/Air Temperature, which usually ends up being within the correct range once you get the Basking Spot Surface Temp correct, or only off by a few degrees. You measure the Hot Side Ambient/Air Temp by using the Suction Cup that came with your Digital Probe Thermometer; just attach the Suction Cup to the wire right below the Probe, and then you're going to stick the Suction Cup to the glass of the Hot Side of the tank, either the front or the side glass, about 2" above the floor for the tank (2" above the floor puts the Probe at Dragon-Head height). Then again allow the Probe to sit for 20-30 minutes, and then read the temperature, which should be between 88-93 degrees f. Again, it should be close, and if it's too hot or too cold within a few degrees, just raise or lower the basking bulb 1-2", this won't throw the Basking Spot Surface Temp off enough to matter, and as long as your close on this temp you're good.
Now comes the Cool Side Ambient/Air Temperature, and I always leave this one for last, because it is sometimes necessary to add a very low wattage, secondary basking bulb in either a dome or a clamp fixture over the Cool Side of the tank, in order to get it up between 75-80 degrees F. KEEP IN MIND THAT YOU NEVER WANT THE COOL SIDE AMBIENT/AIR TEMPERATURE TO GO ABOVE 80 DEGREES, AS YOUR DRAGON MUST ALWAYS HAVE A PLACE TO GO THAT IS CONSIDERABLY COOLER THAN THE HOT SIDE AIR TEMP IS...So if the Hot Side Ambient/Air Temp is between 88-93 degrees, you certainly don't want the Cool Side Air Temp to be 85 degrees, because it would basically be the exact same temperature as the rest of the tank, and he'll have no where to go to cool down. I keep all of my Dragon's Cool Side Ambient Temps under 80 degrees, usually they are around 75-78 degrees, so that there is a definite separation in Air Temps across the tank.
So measure the Cool Side Ambient/Air Temperature the same way you did the Hot Side, attach the Suction Cup to the glass on the Cool Side of the tank, either to the side or the front glass, about 2" above the floor, and wait 20-30 minutes to read the Probe. Again, you're aiming for 75-80 degrees, however, if you get a temp of 70-75 then you're fine, again, this is the side of the tank that he'll go to when he wants to cool down and be out from under the heat, and as long as the Air Temp is at least 70 degrees there is no reason to add a secondary Basking Bulb over the Cool Side at all. 9 times out of 10 the Cool Side Ambient/Air Temp will be lower than the range you're aiming for, and if it is lower than 70 degrees, then the way you control this is by adding a secondary basking bulb over the Cool Side, that will be a very low-wattage bulb.
I build my own wooden enclosure that are usually 4'x3'x2', and I install an adjustable light fixture for basking bulb over each side of the enclosure (I usually use the type of fixtures that they sell at Lowes, Home Depot, or Walmart as "Home Security" Lights), so it's easy enough to throw a low-wattage Indoor Flood Bulb into the fixture over the Cool Side if need-be, but I do also have one very large, glass Exo Terra tank that is about the size of your tank, and it required a secondary basking bulb over the Cool Side, I believe initially the Cool Side Ambient Temperature was around 60 degrees or so with just the main Basking Bulb over the Hot Side of the tank...
Since it's completely frustrating and nearly impossible to adjust or change the main Basking Bulb over the Hot Side in order to get the Cool Side Ambient Temp within the correct range, I simply bought an aluminum clamp lamp at Walmart for $6 (8" clamp fixture), or a deep-dome fixture sitting on top of the mesh over the Cool Side would work fine too...But I didn't bother going to the pet store to buy a Reptile Specialty Basking Bulb, as you have many more options for different wattages if you go to Walmart, Lowes, Home Depot, etc. and buy a Halogen Indoor Flood Bulb. I put a 30 watt Halogen Indoor Flood Bulb over the Cool Side and this put the Air Temp at about 78 degrees. Perfect. Remember, down to 70 degrees on the Cool Side is perfect, but you do not want to go much over 80 degrees at all, as he needs a place that is considerably cooler than the Hot Side Air Temp to go at all times.
You can buy all wattages of Halogen Indoor Flood Bulbs that fit regular light bulb fixtures, actually Walmart sells more wattages than Lowes does, I have a 15 watt, a 25 watt, some 30 watts, 38 watts, 42 watts, etc. than I bought at Walmart in their Clearance Aisle one day, they were anywhere from $0.50 to $2.00 a piece, so i got them just to have them in case I needed an odd-wattage basking bulb...
So that's the easiest way to do it, don't try to get the entire Temperature Gradient correct by using the main Basking Bulb over the Hot Side, if your Cool Side Ambient Temperature is coming up too low (and sometimes the Cool Side is also very dark), then simply add a very low wattage Indoor Flood Bulb over the Cool Side that will put the Air Temp somewhere between 70-80 degrees.