so i don't make a new thread, i need advice on my set up.

I already had my zoomed uth on the tank before I realized i needed to put my tstat probe directly on it. I really don't want the probe in the tank, so I used alum/taped it to the bottom of the uth. The problem I'm having is that it reads about 4-5 degrees lower on the bottom of the uth that it does inside directly on the glass. Do I just test the temp of the repticarpet over the uth inside the tank with my IR, gun and adjust the thermostat so read 90-92 on to of the repticarpet?