Light height

I also forgot to mention that you would really need to use a PAR meter to accurately measure light from the LED. LUX meters measure the amount of usable light for humans while PAR meters measure a spectral range including how many photons fall on the the plants in 1 sec within that spectral range.

Did your LED come with a manual? My LED recommends 36-40" for seedlings/clones, 24" during veg and then 18" during flower. Those ranges are to be measured from the bottom of the light to the top of the plant canopy. In your case, depending on the wattage and intensity, I would put the LED at 24" and adjust accordingly as the plant grows up toward the light. I've let my plants grow closer to 18" from the light in veg and 16-17" in flower, but you have to be careful when getting too close, as the plants can get light burn/bud bleaching and the added heat of being so close can cause some late flower nanners. Definitely check the manual for your specific light. I hope this helps!

I found the best solution for me was to download a Lux meter for my smart phone. I found the specs listed in the user's manual were close but a lux meter really is the way to grow. You can see where the light is the most intense and adjust as needed. I hope this helps and happy growing.