Beth,
You will need a thermometer to check the ambient (air) temps. Your temp gun checks a surface temp not the air temperatures.
In my opinion, there is not a 'magic' number when it comes to gradient temperatures. I think that a low temp of 80 would be fine but I think a low temp of 85 is also ok. I try to shoot for anywhere between these two numbers.
If your lowest ‘ambient’ temp is in the high 90 to low 100’s, I believe that is too high. Your gradient temps should have enough difference between the high (not basking) and low portion to allow your uro to manually control his own body temperature by moving in and out of these temperature zones. If your basket spot is 120 and the remainder of your enclosure is in the high 90’s to low 100’s, he will have a difficult time cooling down when he needs to. I guess it would be similar to you lying in the sun for a few hours, realizing that you are getting way to hot, but not having the ability to get out of the sun. Your body has some wonderful mechanisms for keeping warm and cool (sweating, dilating/constricting blood vessels, shivering, goose bumps); your uro does not have these benefits.
In my opinion, the proper size enclosure is the main key to establishing the necessary gradient. I believe that too small and it cannot be achieved at all. Too large (no such thing with uros), you will just need more equipment to achieve the proper gradient.
Just my 2 cents on the subject
