You can also get a second probed thermometer. So you can have the probe for the thermostat outside the tank, in between the UTH and glass (in the middle - always in the middle). Or, have it inside directly on the glass.
Then, if you use a thick layer or type of substrate, you can get the thermometer probe and place it on the top of the substrate or where you think the snake will mostly be. It just gives you that little bit of extra confidence in knowing the temperature.