Why cant they just make a heater with an accurate built in thermometer. I had to do like a week or two of tweeking with every heater i've ever had. Keep lowering the temp on the heater till it goes off. Then it should be close to correct. Watch your tank temp, make sure it does not go too low. See what temp it keeps the tank when you set it. Ex. if you set it to 75, what temp does it make the tank. If it makes the tank 80, then you know to subtract 5 degrees from the temp you actually want when you set the heater.