Thermostat/thermometer probe placement

Sorry if this has already been answered, I can't find it. I know you should put the thermostat probe on the heat tape/flexwatt in a rack, but where in the rack/under which tub? Will one thermostat handle all three(15qt)/six tubs(28qt) that can be used with it and if so, how do I decide which tub to put it under? Same with the thermometer/hygrometer, should there be one for each tub or can I just monitor one tub's temps/humidity if I'm using just one thermostat? My hubby is hopefully picking up my rack this week. It has heat installed and will come with a Helix thermostat. I plan to set it up as soon as I get it home to make sure everything works right and so I can get my temps/humidity situated. I'm sure I'll have more questions once it's here, but this will get me started anyway. Thanks in advance.

Put the thermostat probe on the middle most tub.

I use a temp gun to measure temps in my tubs. I soldered juts enough holes in my tubs so that there is decent air flow and the humidity stay around 50-60%. I don't check humidity regularly in my rack.

How frequently do you check temps? I would think twice a day minimum? Thanks again!

Honestly, I check them maybe once every other day unless I think something is wrong.