id say start it off at least 2-3 feet away and then gradually lower it every couple of days til u get to about a foot away, any closer than this and the 400w will burn ur plants...but 2 ft is a safe distance ive found especially with 400w lights
...a good way to determine distance is by placing ur hand at the top of the plant, and if it starts to even feel like its slightly burning after 30secs or so then ur lights are too close
..the closer the better. As distance betwen your plants and light increase, PAR watts/lumens decrease. I built a homemade vented hood, and can keep my 1K under 18 inches from the tops with good circulation.