I am going to have to agree with "The Lord" on this one. We do have 2 ARS hatchling racks that have belly heat installed in them. We seem to get a good gradient with those tubs, as they are 22 inched long, which gives ample space for non-heated tub to remain cooler than the heated portion.

However, I do also have 2 racks that I built myself (before I could no longer keep up with snake production and space requirements). Both of these racks use the Iris shoebox tubs which are about 13.5 inches long. One of the racks is 40 tubs (4 x 10) and the other is 60 tubs (4 x 15). We use 2 strips of 11 inch flexwatt (each piece heating 2 tubs wide, with the flexwatt heating about half of each tub) stright down the back. I have the Helix set at 96 degrees on these racks, with an ambient room temperature of about 82 degrees. With this setup, I usually see about 89-90 degrees in the back of the tub and about 82 in the front, giving me about a 7-8 degree gradient in a rather short tub. I think that is pretty good, and my hatchlings have done great in these racks for the last 3 years.

Hope that is informational,