you really don't want to go that low unless you have really good cooling for your lights. the closer you can get the lights to the plants, the better. the distance that's usually given is an average distance for the light not to burn your plants, and this is because of the HEAT the light produces. just place you hand on top of your plants and slowly lower the light until it starts feeling hot (you don't want hot). you will feel the radiant heat. like i said, the closer, the better, but this will also mean better cooling for the light is required.
no air cool = about 2 ft to 3 ft
air cool = as close as you can
i have a 1000 watt that sits about a 1 1/2ft from my plants. everywhere i've read, its been recommended 3 ft. i get some fatties that just don't want to stop growing ;0o
i have a cooler hood and an inline fan that sucks in ouside air through the hood and out the roof. it's really great in the winter...cool ass air keeps my light nice and cool. i cant put my had on the glass of the hood and it's not hot.