Truly it’s whatever works best for you. If possible, I’d suggest trying a few different probe placements for a day or so each and fiddle with the thermostat setting until you find what suits your house + enclosure dynamics best.
In my T8s I have the probe fed through the routered channel for the RHP power cord (I had AP install my panels). I have it hanging about 1/3-1/2 down from the ceiling and set to ~87°. Gives me about 90° on the ground under the panel and 78-82° ambient depending on my room temp. It’s generally pretty steady with the room no lower than 72-73°. If the room goes much lower than 72° the cool side of the enclosure starts to dip below 78°.
Again this is how it works in my room with my conditions. Take a couple days to play around with things and once you find the sweet spot it’s pretty much set it and forget it. Have fun and good luck!
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