Having the flexwatt underneath 1/3 of your tub approximately. This will give your snake room to lay on the warm area, and still move up front where the bottom isnt nearly as warm.

I do use 3" flexwatt on a 28 quart and only use 1 strip (but this is because I use it for two sizes of tubs). I figure an inch or so on either side of the flexwatt should heat up pretty well, so that gives me about 1/3 of the tub bottom for warm basking.