You plants can take more light in flower than in veg. I ran my QB light at 30" in veg while running at 360 watts. A meter will gauge how much light is hitting the canopy but that doesn't tell you where to put the light. And every strain is different. Watch closely for light burn, it starts at the tips of the top leaves. You se the slightest sign, raise your light 4-6" and see if it stops. If your light is as high as it can go, time to bend, tie, break branches!