So, about how far away from the canopy could I keep a 400 watt MH and about how far for a 400 watt HPS?
Depends on your ventilation, etc...I've had a 400 watt HPS 4 inches from the canopy of flowering plants with no harm coming to them, with a big ass fan blowing across the top of the canopy. With my new set up I can't seem to get them closer than 10 inches without burning them. Never grown with a 400 MH so can't help you there.