You should be concerned with your ambient temp and your hot side temp, which (as a rule of thumb) should be arround 80 degrees and 90 degrees respectively. Your snake will use these temps to thermoregulate itself, therefore the snake temperature will not be consistent (like with humans and mammals) but it will usually fluctuate between those 2 temps.
Regarding the hot side temp, you should be concerned with the temp under the substrate, on the actual surface of the tub, since that's the hottest part of the tub your snake can get to.