The cold substrate will only bring you temperatures down a couple of degrees. I have a 40 gallon breeder tank, and I need a 150 watt bulb on it to keep me ambient air temperature at 82 degrees. I would think you would need about the same as I do, as well I use the red heat lights, but you could use a ceramic heat emitter or a black heat light as well. I only suggest those because ball pythons are nocturnal and white light can stress them out if left on all the time. Also, are you using a thermostat or dimmer for your under the tank heat mat?

If you have any other questions just ask.