Yes, the tstat probe goes between the uth and glass, but I was told to place the thermometer probe directly on the glass inside of the tank above the uth in order to monitor the absolute hottest spot the snake could possibly come into contact with, and then to use a heat gun to measure temps of the surface of the substrate.

If that's wrong, I'm sorry I gave incorrect information! I guess you learn something new every day, particularly in this hobby.