Quote Originally Posted by herbie the love bud
With a 400 watt hps you can set the bulb at 2.5' and leave it there the entire growth. From that distance it will provide the max lumens that a flowering cannabis plant can use.
If your HPS is aircooled then you should get it as close as possible. My 2 400w HPS are within 12 inches from the top of the canopy. The closer the better. But obviously don't burn the plants up. Use common sense.

skeet.