I understand the confusion, basically ball pythons are like Murphy's law, anything that can happen will. So you need to prevent it before it does. A hot spot above the substrate (depending on the substrate and how thick it is) can be 90, but then the glass would most likely be well over 100 degrees. Long periods on that can lead to bad burns on the snake. That's why it's safe that the hottest point in the enclosure be between 88-92 degrees, and in a tanks case it would be the glass under the substrate. I'd only use maybe 1/2 inch of aspen that way the heat gets through easier and the readings shouldn't be too far off from the glass to the top of the substrate.