The talk about temp guns vs probes is still one that causes confusion for me and from the sounds of it the OP as well.

Temp guns measure the surface temp of the substrate. Probes measure the temperature of the glass/under the substate. I realize some substrates can easily be moved, but for example I know my snake doesn't try to get under the reptibark. She lays on top of it and compresses it sometimes, but there's always at least 1/4"-1/2" of substrate even at the shallowest points, so does the glass temp really matter?

It seems like the surface temperature is much more important than the glass temperature, especially with something that doesn't lend itself well to burrowing.