I place the probe in the middle and adjust from there.
I tried the individual thermometer way and it just got to be to many thermometers, I went and bought a nice heat gun
I set mine up for about a week before I put any animals in, I usually set the thermostat around 100 and then work from there. I use back heat on my racks, with belly heat you could probably start a bit lower.
I keep boas and do not know if the BPs still need hides or not. I do have some boas that still like their hides in the racks though, once they grow out of the racks and into their cages they do not get hides anymore.
Hope this helps
* Very nice rack by the way