Where to measure temp of hot spot

Ok, I am setting up a new enclosure for our girl Luna in my classroom. I was wondering where people measure the hot spot? I am been measuring the temp under the substrate right over the pad. Should I measure here or on top of the substrate? There is a difference in temp. 90 on the plastic and 87 on top of substrate. I know both are fine, I am just wondering where most people measure their temps.

The correct place is the hottest spot your animal can come into contact with.
Seeing as any snake can and will burrow, you want to measure under the substrate.

That's what I have been doing which I figured. Otherwise that hot spot would be to hot. On another note, this girl will be returning to my classroom. My students have been asking when she is coming back from winter break. We had a heat issue in the building where my room temps were getting down into the low 50s at night, but that seems to be corrected for the time being. I am a little worried what would happen if that were to occur when she was here. Would the snake be smart enough to stay on the hot spot? If it were to happen again I would bring her back home, but I am still worried what a night like that would do to her.