honestly i keep my 600watter about 10 inches away from the closest tops and they do great for my 400 watt i keep it no less than 8 inches away. but if you do it that close you better be checking them every day as mine will grow few inches a day so your 8 inches could turn into 5 or six real quik and then burn baby burn burn
happend to me yesterday i didnt check on them for 2 cycles got a tiny bit burned