A half pound is 224 grams, and the formula only counts HID light, and only counts it in the flowering room... you can add a LITTLE for flouros but not much, I'll give ya 200watts.
With a 2 month flowering strain, this is your GOAL:
((0.5g/200w)/30d)
So you have 100g per month, and a 2 month strain, that gives you 200grams total, and that is actually considered a level attained by an experienced indoor grower with everything dialed in perfectly.
Anyway not to argue about that...
Your problem is that you don't have 'real' ventilation. A passive intake (=crack under door) can't handle the heat from HID lighting.
You're going to want another exhaust placed high up... oh fuck it, there's someone doing this exact space, here's a link to her log....

http://boards.cannabis.com/grow-log/...ck-game-p.html

She's growin in the same size walk-in closet and has the capacity to flower ~ 50 plants finishing at ~1-2 feet each under 2000w total. The plants have to go in as 10 inch clones, rooted and then vegged for a couple weeks. She isn't finished getting her clone production up and running so the plants that are in there aren't quite the way they are eventually going to be, and she's only got one of the 1k HPS turned on atm.