The basking spot temp, directly under the light, should be between 100 and 110 degrees F.
The rest of the cage should be basically be in the 80s. Ideally, you would have a gradient going from high 90s down to low 80s.
Is some of your viv ends up being in the high 70s, that's fine, as long a most of the viv is in the 80s.
To check the basking temp you need to get a digital thermometer with a probe or a temp gun that lets you point it at a spot and get the temp on the surface.
If you use a digital thermometer with a probe, then, without the lizard in the enclose, you need to set it up, turn it on put the probe on the basking spot, turn all the lights on, and let it sit undisturbed for about an hour to get a really accurate reading of where the temp is going to max out and level off.
Do this same thing at 2 or 3 other points around the viv to get temp readings at each of those locations. Ideally, directly under the basking light, away from the light on the same side as the light, the middle of the tank and on the side opposite the light (the cool side). The "cool" side should be somewhere between 78 and 85 degrees.
