You need to get a lux meter, they measure how many lumens are being put out by your light. You take a reading with the meter at the top of your canopy and then following the guidelines people have posted you can dial in your light distance. Here is a link to a lux meter Hydrofarm LG17010 - Digital Light Meter - Up to 5000FC Sent from my iPhone using Grasscity Forum mobile app
I second the use of a light meter. I just got one last week. It's really easy to use and really helpful when arranging my plants under the lights. I had my LED way too high for my plants to benefit from it. This one from eBay is way cheaper than he Hydrofarm meter posted above, which has pretty bad reviews. eBay Sent from my iPad using Grasscity Forum mobile app