The thermostat probe needs to be between the mat and the glass. The snake will just bulldoze the probe in the tank and get it wet and it will end up failing. You need a heat gun to check the temperature on the glass once the thermostat is set up and has been running the mat for a couple hours.. And yes, you might have to set the thermostat higher than 90 degrees to get the temperature right on the heat gun on the glass.. One other thing.. If this snake is young you will want to be really careful about constantly messing with his cage like you are doing.. It's a big tank for a little snake and you might run into feeding issues if you are constantly messing with his set up...