this is a good question, and one w/ many answers. i think it has a lot to do w/ your hood or reflector. it also has to do w/ the strain. some are heartier than others. i would start w/ them 20-24 in away, then see where you stand. i have air-cooled lights and i thought i would be able to get my girls very close to the light. wrong. anything closer than 18 in will start to slightly yellow(burn) my top. i use thousy's though. how cool is your room?? hope this helps.