Put the probe on the outside of the tank between the bottom of the tank and the heat mat. The top of the substrate will be a little cooler but that's ok as if the snake wants more warmth, he will burrow down or push the substrate out of the way. My little girl does that, she pushes the substrate aside inside her hide and just lays on the glass which I keep around 93-95 degrees depending on time of day. Cool side is just kept at 80 degrees. She bounces back and forth. I just wouldn't let the glass get over 100 degrees though. Temp guns also make it easy to check substrate and temps.