i veg and flower under my 600w hps i put my youngsters under them asap whether seedling or clone it doesn't matter just start them with the light a few ft away (3ft) and lower the light on them as they start to grow, i drop mine a couple of inches every couple of hours and then stop when they're about a 1ft to 18inches away you should be able to get them alot closer than that but it's not needed especially for seedling/clones, i've run 18/6 and 24/0 with them in veg and prefer 24/0 because of closer node spacing but it doesn't make a lot of difference apart from the electric. one thing i would say is to make sure they have a fan on them and with good conditions you should get really fast growth under your 600. typically my plants get to about 18inches-2ft in a month of veg and look super happy and healthy under them, people will say about using cfl's as they work well and i use them aswell but they don't compare with the 600's
