I have a ball python in a 40 gallon glass tank, and Im having heat/humidity issues.

Im switching around CHE and a zoo med UTH.
I think I've almost got the heat narrowed down.
Useing UTH on basking side and CHE on cool side.

My question is, how should I measure the UTH?
Should I put the thermometer probe on top of the substrate (in my case Im using repti-bark). Or should I put the probe on the bottom of the tank?

Its 106 degrees on the bottom and 78 degrees on top of the substrate.
Is there something Im missing here?