The substrate over a uth should be no more than 1/4 inch.

You want it thin so you don't have to ramp the uth up to reach the desired temp on top of the substrate. You also want to monitor the bottom directly over the uth as well as the top of the substrate. You do not want any portion of the cage to reach a unsafe temp. They can and will burrow and having a thick layer of substrate sets up the situation to put your snake at harm.