The temp will get lower as the heat rises through the bedding, but get the temperature where he rests on the substrate, and the floor will be way over 100 degrees. Which if and when he burrows down into the substrate, will most certainly burn him.

Put the probe for it under the tub, on top of the UTH. Use a thermometer inside the enclosure at the bottom of the substrate. Still with me?

Get THAT temperature to 92 to 94 by adjusting your thermostat accordingly. If he needs to be warmer, he'll burrow down and get to the heat. Simply put, NO spot in the enclosure should be over 94 to 95 degrees.

Sit back and have a beer