Yeah, I heard a guy that owned a grow shop once say "not enough light is NEVER a problem. Too much light is ALWAYS a problem".
I wouldn't necessarily say I agree with him; he said to raise your lights as high as they would go, and keep them there. Even if its like 5 feet. Having your light too high can cause stretch, airy buds, and just a lack of yield in general.
There's a graph somewhere on here that shows the closest you should keep your lights, for each wattage bulb. I think it says 18" for a 1000 watt and like 14" for a 600 watt.
The hand trick works great, just stick the back of your hand under the light at canopy level and leave it there for 60 seconds. If it starts to burn, then you know it'll cause problems.
Personally I keep my 1000w about 30" away at the beginning of flower, and try to keep my shortest plants right under it. I've had problems before, where I put my plants in the flower tent under the 1000w and the tops wind up all getting real yellow, and sometimes taco-ing up on me. I'm pretty sure that was happening from too much light, so I moved it up from 18" to about 24, now i start at 30 and bring it closer as it gets closwr to harvest, like an inch a week so its closer to 20" by the time they finish.
