75 gallons for a ball python is huge...is it full grown?? I suggest switching to the UTH and thermostat asap, it's much easier and less stressful for the snake. The hot side should be about 88-92 and the cool side between 75-80. How are you reading the temps?
Oh, and here's the caresheet, it goes into more detail
http://ball-pythons.net/forums/showt...ius)-Caresheet
Oh, and as for the size of the UTH, it should cover 1/3 to 1/2 of the bottom of the tank.