To append my previous post....IMHO a rheo should never be 100% trusted, they are a band-aid solution. Like a splint on a broken leg instead of a cast. I would start thinking about a long term solution. Save up for another thermostat, or somehow become flexible in the lengths of your flexwatt.

It depends on how your rack is built, but if you have 12" and 24" strips, would 16" work? Could you change your 12" strips for 24"? If the tubs are only 12" wide for example, the exposed flexwatt is just heating the air (assuming wider shelves). Personally I would rather have exposed and unused flexwatt (that essentially does nothing other than waste power) than a rheostat.

Just some food for thought.