When you first put the plants under an 600w HPS, I would keep them at least 18" away (probably more). Every day i would lower the light an inch or three and keep a close eye on the plants for signs of heat damage (like very 10 min. or so for the first hour). Eventually you can get them pretty close. You can get them real close if you use an air-cooled reflector with glass enclosure.
A good rule of thumb is if your hand can stay under the light (where the tops of the plants are) and it is not uncomfortably hot, your plants should be OK. But do keep a close eye on them at first.