Minimum distance to keep hps light away from plants

[Shadowhunter]

I just got a 400W HPS light that I have started to use for flowering.
I would like to know what the appropriate distance to keep the light away from the tips is? I have read online anywhere between 12-18 inches can anyone confirm? Thanks in advance.

-Shadow

 

[Lord Jebus]

Wea are using a 400 watt HPS conversion bulb, and have it less than a foot away from tallest top and no problems yet, got a fan blowing right across the canopy. Good rule of thumb is put your hand at the canopy, if your hand is hot, the plant is hotter.