Originally Posted by Socratic Monologue

Well, unless the snake buries under the substrate, then the reading isn't all that helpful.

IR temp guns are the way to go: cheap, non-intrusive, accurate, can also be used around the house for other purposes. They can also measure the temp of the substrate, temp of the snake, temps in the rest of the cage, all instantaneously.

Your thermostat will help with all this. You likely still will need to calibrate the temp the thermostat probe reads with the temp that the snake experiences (because unless the probe is taped to the snake's belly, likely the pad will have to get warmer than the temp you want the snake to be able to maintain).