how close?

how close should the top of my plants be to my 400 watt hps. bulb?

These distances are only possible if you can control the heat:

Flourescents: proximimty
400w HPS: 1 foot
600w HPS: 1.5 foot
1000w HPS: 2 foot

with a 6" tube cylinder i can get about 8 inches from 1k with lots of room air and 6 inch inline fan

the more light you give the more nutes your plant can process if you like 16-24 inch colas go for 1k.