Wattage plays the part of how hot the tape can get. I like covering roughly 30% of the floor with heat tape of whatever container im using. So 12qt gets 4" and 28qt and 41qt get 11". Maybe its too much but my babies eat and my adult male eats and there all very comfy in there tubs this way. This is how i do it though. Theres many different ways to use flexwatt and none of which seem wrong if all applied in the same fashion.