Use your back hand to see how hot it gets, if your back hand get uncomfortable then it will be uncomfortable to the plants too.Find your sweet spot where is feel's nice to the back hand, but close enough so where the light is low enough to get the best lighting.1 to 1 1/2 feet works best.
I find with my air cooled hood, I can get my closest buds to 8-9'' from the 1000w light, but doing it that close, I have to watch the light all the time to not burn the plants, or buds when the take off.