I am also looking at a RBI 3x2 cage with RHP. I talked to some guy there...he said depending on how cold ur place gets you can either get a 40 or 80...hold on..I'll just copy paste the email he sent back to me.

"I have had some customers in the NW USA with a similar problem. The 80 watt rhp is overkill for the summer months but will keep the enclosure nice and toasty in the winter. I would run the probe through one of the vent slots in the back of the cage. You will have to tinker with your thermostat settings but once you get it right you will be all set. By the temps you are giving me I am guessing you have a ball python? Your gradient should work out just about right but will ultimately depend on the room temps.
Rich"