I'd say it all depends on your particular setup and other factors like that. You'd have to try it out and test it and see what's right for you. For instance, my flexwatt has to be around 110 degrees during the winter to heat my hot spot to 92 degrees. For others, it'll be different, depending on how close the heat tape is to the tub, how cool the room is, how much substrate, how thick the plastic tub is, etc, etc.