Kinda goes without saying, but you have to temp gun the surface when the thermostat is on, and ideally, right before it clicks off to see what the max temps are. When my UTH turns off the hot spot will drop down to 80-84 degrees before it pops back on.

You also have to be careful for inconsistent UTH's. You can get hot spots so if the probe is on a hot spot it may results in lower temps.