As long as good air movement that environment will grow just fine. I sit at lower humidities all the time. Just have to water more. Also as they grow humidity will rise. A little warmer during lights on will stimulate more growth. 80-85 is also a perfectly fine temp. These statements come from my personal growing. I quit chasing humidity if low and only stress when high myself.
Good suggestion
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As said white paint has nearly the same reflective value, BUT lining the closet with something like mylar emergency blankets provides condensation and humidity protection to the walls as long as it’s sealed up properly… Depending on the location of a closet the high humidity in a closet like say next to a cool outdoor wall or even a AC’d room could condensate and cause damage to the drywall in time…
IMO, yes, small gentle breeze helpful in life.
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Nice, I like it. Does air come in under the door and then out the ceiling to the attic?
I might consider a shelf that goes all the way across. Nothing fancy needed, just plywood on 2x4s maybe? I feel like you’re going to want every bit of horizontal space and the rack may not do it. If you did that 2’ up (or whatever makes sense), you might could put a veg light under and let the next gen come up to 3-4 weeks below and then graduate to finishing school on the top floor too. Maximize the vertical space as well. IDK, just a thought and others may know better, including you 
And +1 re: fan