when to move light closer??

hello, i have 2 large sprouts, about an inch and a half in length, 1 medium sprout, around half an inch, and 2 more that are barely out of the ground. the bigger ones look like they are already starting to stretch, and i have my 400 watt HPS system 3 feet and 3 inches from the soil. how close can i get to my sprouts? i dont want stretchy weeds. but i also dont want to burn those little puppies out either... thanks...

you are bound by genetics and shear bad luck to have some plants that grow taller/quicker than the others. When this happens try to still maintain the proper distance for the HPS lamp to the top of the biggest plant. It is easier to put wood blocks or something else UNDERNEATH the smaller plants to raise their peak than it is to shrink the top of your big ones through telekenesis.