I got this tip from Kevin McKurley's book and it's saved me a major hassle. The idea is you need air flow on one side of your heating tape. Just laying the tub down on top of some flexwatt is not good enough, even if you've taped it down securely and evenly to either the tub or the surface below it. You can fix this by using some of those little felt furniture pad in each corner to raise the tub up about a half inch or so, all around.

This little tip has completely fixed the setup problems I was having, which included surface temp variation (aka Hot Spots) and not seeing enough of a heat increase on the cool side. Now I'm able to keep my temp gradient at a near perfect 80 / 90 with only 1 heating element inside a 74 degree room. The warm side is also much more evenly read by my temp gun.

Can't recommend doing this enough. Thought I'd share..