Starting Under 600w MH Safely?

At what distance should I start it out under my 600w MH from seed? Should I dim it to 50% or would that make them stretch? I'm doing hydroponics and they're in 1" rockwool cubes right now.

Anyone have any input?

What type of reflector do you have? Mine is a 600w enclosed/air cooled hood so I can hold my hand on the glass when my light is on all day and it's not hot. Just depends on the type of hood, if you have just a typical reflector I wouldn't get them any closer than 18 inches which is going to make them stretch. I veg my plants under T5 cfls and flower under the 600w HPS.

I have a cool tube and it doesn't get hot at all really, I just didn't want to fry them up with too much light or something, because I've heard they're too strong for seedlings.