[Cougra]

Personally I use a 5 gallon bucket to set up heaters. Set the dial to about where the temperature I want to keep the tank at should be and place it in a bucket of water with an airstone for a bit of circulation and leave it couple hours then I test the temperature in the bucket. If the temperature is too low then I turn the dial up a little but if it's too warm I change out part of the water to cool it down quicker and then turn the temperature down on the heater.

I don't like keeping the heater in the tank while trying to adjust the temperature because it's too easy for them to cause temperature fluctuations. It may take a while to do set it to the desired temperature in the bucket but at least your tank will be safe from temperature fluctuations.