I use 11" flex watt on my 28qt rack with no issues for the past three years. I just offset the heat and the tub by either runinng half of the heat tape on the bottom and the other half under the tub... or with most of my tubs i have, i can litterally just put the tape under onlt the section of the tub i want to heat.

With that being said, 'i do use 4" under two of my tubs and it works too. the 4" provides a good hot spot but the 11" helps out the whole tub temp gradiant better.