you need a thermometer inside on top of the substrate to measure the temp of the hot side above the UTH and adjust your thermostat to get the temp up. say your hot spot is 82 and uth reads 92. you may have to adjust the thermostat to 102 to reach temps of 92 inside the tank on top of substrate. dont quote me on the thermostat setting just play with it a bit till you find the right comparison