When you use paper towel or any paper as a substrate you want the bottom of the tub to be your target temperature. When the snake is on the papertowel the heat passes almost right through it. So when you snake is on the papertowel he gets the 110 degrees.

I would also check your snakes body temp after he has been on the heat for a little while. Non Contact IR Thermometers are the best for checking body / surface temps