not sure if I can be 100% simple on this but I will try.
you need to figure out 2 things.
1 how large of the tubs you will be using. as well as the heat tape. (the tape imporant since if using something like 3 or 4 inch tape might need to run a second strip to get a large enough hot spot for the snake)
2 I would use a tstat or something as the dimmer may not control it it only limits the elelctrical current to the tape so if you have set were you want it during the 80 degree day by that night or the following day must adjust it every day to get the temps to sit stable.

But the simplest way to set up flexwatt( heat tape) for me is to do it per shelf so when a shelf not being used it can be unplugged. This is were those power strips come in handy as I have mine set up to handle 6 shelves at once. I set my flex watt about 4-6 inches from the back of th rack to ensure that it all under the tub(s) since some tub have the hande/ lidsthat stick out some that makes gap along the back (and back heat in a rack a poor choice) ithen figure out how I want to afix the tape to the shlef surface( me I used something like liquid nail ) and a paint straper to make a thin spot for it to stick too. I also use some bath tiles to creat a spacer between the tubs and tape now since it seems to wear from dragging accross it a lot. found taping the probed on top of a tile that only half on the tape gives me an safe reading for the tubs.