if i fan a 400w mh how close can it put the plant?

enclosed space with a small fan blowing on it

loser..........

Stfu stonerdude. Keep it like 15" theres a chart. Let me find it

Here it is
http://www.google.com/m/search?site=images&source=mog&gl=us&q=mh marijuana chart#i=1

12 or so inches

it depends on your RH, and the temp of your room without the added heat of the lamp.
start at 15 inches, and drop slowly till you see signs of heat stress, then bump it up 2-3 inches.