You want the thermometer probe directly over the heat tape, regardless of where your hide is. I'd suggest using blue painter's tape or hot glue to hold it down. Both are safe and won't cause any harm if the come off.

You need to take one of the drawers out of the rack and measure the distance from the back of the rack to the back edge of the heat tape channel, then transfer that measurement to the tub, and put the probe about 2" forward of that mark. That'll ensure that you're getting good measurements.