Okay, put the heater and a thermometer in a bucket or a fish tank without fish in it. Set the heater to the temp you want. Wait a day or so and look at the thermometer.
If I set my heater to 79 degrees and after a day or so my thermometer shows 83 degrees, I can generally assume that I need to set my heater to 4 degrees lower than the temp I want the tank to be at. So If I want the tank to be at 80, I would set the heater for 76. This process is called calibration.
This only holds for a tank of that size, however. The temp I set my Hydor Theo 25 watt heater at to keep my 1 gallon tank at the right temp is a bit different from what I set it at to keep my 3 gallon tank at. You may need to re-calibrate at different times of the year, especially if the temp of the room the tank is in can vary a great deal (ie, my living room here in the mountains without a/c is always in the 80's-90's in the summer and in the 60's during the winter).
Does that make sense?