Don't worry too much about humidity.
As long as he has a full water dish, the sub can be relatively dry. At around that size they begin to develop the waxy outer layer on their exo that prevents excessive water loss, so they can be kept drier than slings.
Yes the TKG is the book Carlos mentioned, and the author is the same guy who I'm sure I've linked you to earlier in this thread, maybe? I don't remember and I'm too lazy to look, hahah.
Here's a relevant link from his site, just in case: SPIDERS, CALGARY: GROWING YOUR TARANTULA