It's hard to give a measurement for how far away your light should be, basically it all depends on your temps and air circulation. I'd try to start at about 18-24" if your temps allow (no higher than 80F at the tops of the plants) then try to get it lower over the next couple of weeks if you can, be careful tho, keep your eye on the temps and look out for signs of heat stress.

Yeah i've read your passive intake should be twice the size of your exhaust, my grows in a tent and for most of the time my passive intake is the same size as my exhaust but if it gets a little hot i can open another vent in the tent and the temps drop a couple degrees.