is in on the inside or outside of the tank?

substrate will keep some of the heat from reaching the surface (depends what type of substrate and how much of it there is) but your snake can still dig down and get burned. You do have a second heat source right?

Be warned that the thermostats sold in pet stores are no good, here are 2 that are

the hydrofarm thermostat is a good starter thermostat f you are on a budget, but you should plan on replacing it with a better thermostat when possible

http://www.amazon.com/Hydrofarm-MTPR..._bxgy_ol_img_b

The herpstat line of thermostats sold by spider robots is the best you can get, they cost more but do a much better job and are safer.

http://spyderrobotics.com/

What are you using to measure temperatures/humidity?