Very young seedlings 6-8 inches. After a few sets of leaves appear they should be in close 1-2 inches from foliage. This is very important with florescent lights.
i keep mine abt 2-3 inches jus coz sumtimes they look a bit hot only the leafs near the bulb tho, over all room temps are good
envirolites and ecolites get a bit hotter
i keep my CFL (40w 4100k) about 1 inch away sometimes a little less or a little more depending on how long im going to be away from the plants.
and CFL can and will burn the plant if it touching for a little while