HPS lighting, distance and number of plants

I just finished my first grow with CFL's and it went well but I have decided to go with HPS/MH. I purchased a 400W system with a vented hood, and will be using a 6" duct fan to keep the light cool. How close can I put the fixture to the plant when using my MH bulb, or my HPS bulb? Also how many plants do you think I could cover with the 400W system. Supposedly the HPS puts out 55,000 lumens and the MH conversion bulb puts out 36,000 lumens. Even though the light will be air cooled/vented, I'm a little concerned about burning or killing a young plant. Thanks for any advice.

around 18 inches i think...hold your hand at the top of your plants or a thermometer to guage the heat at the plant

Yeah thats prob good for young plants. altho with my vented 400 and a shitty 6 inch duct fan i got the light to 8 inches all wee so far. The temps were cool as 70 outside and with a fan the light only get a few degrees hotter if yu go tthe right space! I got a 4x4x6.5 tent.

Thanks for the replies.