We put the t-stat probes snugly between the UTH and the bottom of the tank. If the probe is inside the tank the snake can (and will) move it, which can cause your UTH to get too hot. Once the probe is in place you need to use a temp gun or IR thermometer to measure the heat on the top of the substrate hot spot, while figuring out where to set the t-stat. In enclosures where we only use newspaper as a substrate a t-stat setting of 92*F typically creates a hot spot of 89-90*F.