Do the hand test. Pot leaves are about as heat-sensitive as the back of your hand. So place your hand on the top of the plant under the light. If your hand is uncomfortably warm you are too close. I've had a lot of experience with a 1000W set-up and know that most strains need to be 18 inches or so from that big bulb. But do the test and make sure. First thing to watch for is pistils wilting and the leave tips turning brown. Good luck. BigSteve the OG.