honestly people will tell you to keep your light as close to your plant as you can but it depends on how powerfull (lumen) it is (to prevent bleaching witch requires a fuck load of light as i understand it) heat and focus

now asfar as bleaching goes i have only read about it a couple of times and dont know how much light you can rock before you bleach and stunt growth

heat i know about and ya wanna try and keep the top of ya canopy about 80-85ish tops 90's will stress ya plants and about 110 will out right kill em (had it happen once outdoor in the desert outta 12 4 were female 4 went hermie and 4 outright died during a heat wave)

and as far as focus ... ya ever play with a flash light? ya wanna make sure ya got a uniform coverage of the grow area. no dark spots.

another thought is light degrades by about half by my understanding for every foot it travels so it is important to keep the light as close as possible asuming you have your heat and coverage in check.

but hey do some research im not guarnteeing this information cus im pullin it from memory/ass and dont have any refrences hopefully some one else can hook ya up with another oppinion of the situation.