The probe for the thermoSTAT goes directly on the heat tape. When you are measuring the temps for the hot spot, you should put a thermometer inside the tub and adjust the thermostat until its 90-94*.

Is the thermometer a probe thermometer? I would say that would be pretty acurate for measuring temps.

It is a lot easy to shoot temps in racks with a heat gun though. That way you don't have to have a thermometer in each tub. Just check with the heat gun once a day or so.