First of all, set the differential to 1, not 2. That's how many degrees off the 'set' temp it will go before turning on or off.

Second, there will often be a difference between the set temp of the stat and the actual temps in the tubs. My Herpstat is currently set at 102 to get me 92-94 on the hot sides (tub temps also vary with height in the rack). Rely more on the thermometer and temp gun readings in the tubs than on the stat setting.

If you hooked everything up right, the flexwatt should be ok... try tweaking the settings on the Ranco and see what happens.