Okay I ran into this problem a couple of days ago. The most important temperature is what the floor of the tub temperature is. Your heat tape is going to be way hotter than the actual temperature of the floor of your tub. I have a herpstat 2 that is currently set at 107 to get a 92 degree hot spot on my tub; my heat tape is show hotter than that. I have received conflicting comments on whether to attach the probe of the herpstat to the heat tape or the bottom of the tub. I highly recommend purchasing a heat temperature gun.