I would measure where the hottest place is, where the snake can get access to. If you got it at 90f ontop of the substrate, it would be alot warmer under the substrate and the snake will eventually get there. They like to burrow and will move stuff out of the way. I take all my substrate out, measure where the heat tape makes contact with the tub and meause from inside the tub.
Sent from my SAMSUNG-SGH-I747 using Tapatalk 2