stick to the rule of wrist.... wait i mean thumb lol
Put your hand between the light and the plant, is your hand hot? if so it is too close. The usual rule is to keep a HID light 1-2 ft away from the tops but if you got that light cooled enough you can get it as close as you want, as long as it is COOL ENOUGH.
this may or may not answer your question:How far off the plant would be safe for a 600w hps? it will be in a cool tube.
AWESOME! My ladies are getting 200% !!this may or may not answer your question:
https://www.rollitup.org/attachments/general-marijuana-growing/247122d1227270612-question-experts-hpsgraphpar.gif