well since the probe is measuring the flexwatt directly, the temps in the tub are going to be alittle less. so just keep raising the temp on the tstat untill the temp inside is what you want.

example. my rack uses flex and i have the probe taped directly to the flexwatt. i have it set at 104˚in the middle of the rack, and the temp in the top tub is 92˚, and bottom tub is 88˚.