oh I just noticed you said you already have the rack built.
If you are running it lengthwise along the back (reccommended because if you heat the back, and it is a closed rack, it will stay warmer in the back, and cooler towards the front as that is the area that is near the outside room) then you should use 2 strips of 3" spaced out a bit so that you give about 1/3 of the tub as a basking area....
at least thats what I would do.![]()