When I set-up an enclosure I size my heat sources to cover at least 1/3 of the floorspace but not more than 1/2. That's just my preference. My 60qt tub has a 11 X 17 UTH under it so about half is covered. One of the biggest factors that affects the cool side temps is the ambient temp of the room itself. Since I use a space heater in my herp room to keep the temp at 80F, the cool temps in my tubs normally varies between 78-82. I've never used Flexwatt, so I don't know the specifics about cutting and sizing it and this may or may not be an option, but I was thinking you could start off with about 2/3 of the floorspace covered and monitor the temps for a day or so. If it's too much, then trim it down a little bit at a time until you get the desired temps...