How high above marijuana plants should lights be?

A question from a fellow grower:

I bought a couple of 400 watt HPS lamps . I did not see in the article how high they should be suspended over the plants

Anywhere from 24"-32" most likely. Judge with your hand.

With hand held at canopy palm down. PLace the lamp at a height whereas your hand is comfortably warm, and not hot. If the back of your hand is hot, raise lamp 2". If too cool, lower 2"