Lighting/Ventilation for Dresser Stealth Grow

Hey everybody.

I'm setting up a dresser stealth grow. The dimensions are 2.5 X 1.3 X 3.8 ft (L x W x H).
I plan on having 2 strains growing at the same time using a DWC hydroponic grow

I plan on using 250 watt HPS and MH lights.

I was wondering what kind of ventilation I would need?
Not quite sure how to calculate CFM needed for the exhaust, I plan on using a a homemade carbon filter too and heard that it reduces the power of the fan by 50%?how large would the hole for air intake need to be?
and how close should the light be to the canopy and how close could i get it?

Do not under estimate the heat from a 250w hps, its more heat then light ...just maybe here I'll go out on a limb and suggest a low end(Chines) LED ugh...but they are less heat han a hps