I have the 6 tub rack, and I taped my probe directly to the heat tape on the bottom shelf (kind of on the side/corner where it curves up to weave up to the next shelf).

Right now I only have snakes in 3 of the 6 tubs, so I have snakes in the top 3 tubs. It may be overkill, but everyone says that the top tubs are usually a little warmer than the bottom ones. So, I have an acurite in the top tub, and then the bottom tub (actually the fourth tub, since the bottome three don't have snakes in them). I then set the thermostat so the top and bottom tub stay between 90-92 degrees.

I hope this all makes sense!

I am sure you will love your new rack. I have had mine for almost a year, and it is absolutely great! The only thing that sucked was the long wait!

Cheers!