From what I understand, he hardens off all his plants so it doesn't matter what the conventional meathoid of growing is. (sort of along the lines of how I grow dark VFTs when needed)But, I don't know if he has any northern or tropical pitchure plants which usually require humidity for maintaning their traps.
As to a formula, you have to take into account tempurature and air movement, but if you really want some numbers, I actually have some, lol. *stop looking at me like that, it's from my chem class okay*. I'd rather not have to go dig out my notes, but I can if you still need them.