I have Phoenix in a 20 gal long, I also use a rock as a basking spot.
Rocks hold in a lot more heat and make a hotter basking spot, that's why I can get away with 75 watts.
You need to put the probe right where he's basking... That way you get a temp accurately of what he needs.
Measuring off to the side will get a temp there, but that's not where your beardie is basking.
Kach7, here are a couple of pics from the viv I built years ago. The two round lights are par20 halogen floods, 50w each. I have since changed the one in the middle to a par16 halogen 35w flood. The socket on the far right of the viv's lid is a night-light that I wired, but really don't use. All these halogen floods are availabe at Lowes, Home Depot or the evil empire...small-mart. The florescent tubes are Reti-Sun Zoo 24" uvb 10.0 bulbs, only from a reptile supply center, pet shop, etc. Temps in viv range from95-100(+), 102 at highest. I have also gone to a slate tile floor, no grout for easy tile removal for clean-up, etc. Had originally used crushed walnut shells, but way tooo dusty and really it becomes a litter box after awhile.