Julian - I'm just following the advice of those that have come before me! LOL
I got the incubator from Justin, and he didn't put the thermostat probe inside the egg boxes - so I'm having fun with the "heat rises" part of this with the top large tub (if I stay with three of those) being 5 degrees warmer than the bottom one. But then, I also measured with the Acurite probe with no incubation substrate - not sure if that makes a difference.
I've also filled the bottom section of the fridge with 20 water bottles to help keep temps stable when opening the incubator (yes, they all fit!) LOL