I feel you don't have one yet order a infra red temp gun from Amazon. You can get one for about 15 dollars. I move substrate and check the temp on the floor of the enclosure, my hot side thermostat sits at like 94 to keep the floor at 88. My cool side is pretty much dead on, point is don't trust the thermostat reading to set your heat.
Also I might have missed it, I think you said the snake was 2 years old, have you had it that long or is it new to you?