If I run my light at that height (10") I burn the crap out of my leaves. a 600w could be a little closer than a 1000w and if you're not experiencing light bleaching, then it's all good. Just remember, more light = more nutrients that are needed. So if the intensity of the light is higher (read: closer to the plant) then the nutrient intake will be higher. Just take baby steps so it doesn't overdo it.