Im not sure about your calculation there chief. How come you divided by 5?

Either way...for your ballast it depends on what kind of housing you have. To go the cheapest route you would just get a bat wing style reflector to mount your light to, and then do a homemade cool tube. If you google or search the forum you can find more info, but basically you use a glass candle tube that fits over your light, clamp the ends and attach insulated ducting(insulated means less noisy with the fan(s)). Then you have a fan pumping or sucking air.

Oscillating fan is good, and for the light proof intake fan just look for a light trap construction. Its basically a fan mounted to a maze of cardboard or wood (or w/e) that effectively traps light by using the 90* turns.


edit-I had a drawing but it didnt work, think of an s curve...just google light trap.

edit2- Just saw the sketch, you will need a carbon filter on the ballast air supply if you are sucking air from inside the grow room, just keep that in mind. If you pass air THROUGH it (external air, so from one side of the tent to the other) you may not need a filter. Just a thought.