On/Off Thermostat is what I use and again, I have a 1FT tall tank. With the thermostat probe 7 inches above the ground (approximately), and only about 2 to 2 1/2 inches below the RHP bottom (where the heat comes out), in Behira's tank, it's set to 82F to keep it 86-89F on the ground.
If using a proportional thermostat, like a Spyder Robotics, I would set to 83F and see where you are. It should take 10-30 minutes, tops, to dial in a tank that size, if the RHP is the correct size.