is that a aircooled hood? with that amount of wattage you almost need one in that little of a space unless you have really good intakes and exhaust.... are you running passive intakes or fan intakes? im not sure i would put a 400 watter that close to the plants ... i would put it at around one foot or maybe as close at 6 inches.... with my 600 watt in a air cooled hood i leave it at about 14 inches above the plants.... its different with cfls... u can almost grow the plant into the cfl without burning it