agreed on the probe setup with albert. Also use your thermostat gun to check the temps of the actual tape to see if the tape is heating unevenly. A rack I built a long time ago had a back that wasn't flush with the shelves to the heat was traveling up so each tub up was warmer, but your saying its cooler in the middle, which makes me think your having an issue with the heat tape. if the tape is registering different temps then call the rack builder and he may send you out a new piece of heat tape for free. As far as Flexx Watt vs THG, I have used both and found the THG to not be as consistent (which isn't what you hear most).