light distance

You should measure from the top of the plant. I recommend attaching a string to the lamp, with the length equal to the desired distance. This will help you to place the lamp in the right distance.

That guy nailed it. canopy level.

When people discuss this, it is measuring from light to top of canopy. You should hang your light where you get the highest average umol/s of PAR throughout canopy -- at top. With ambient co2, shoot for 35-40 DLI (daily light integral). if your light specs are accurate you can guesstimate the coverage it should have. This puts you in a fairly small ballpark. You can then use a klux meter or phone app to measure intensity of light. Set a distance that covers your intended area at end of flower with as little difference from center to edges as you can without sacrificing overall average. Proportional intensity will relate to umol/s 1:1. If it is 50% as strong in the corner, that's 50% of umol/s of PAR too. Due to how light spreads out, you get greater penetration of stronger light from a light source that is stronger and further away (inverse square law). This is not all gravy. It quickly becomes very inefficient to jsut keep addingd power and distance, obviously. With today's led grow lights somewhere between 16-24 inches will cover most things from bar/strap shaped frames to quantum boards. QBs need a bit more distance, in general. The better the diiodes are distrubuted, the closer the light will likely hang. but if you ahve a choice of 12" and 80% or 16-18" at 100%, i'd go the 100% route for reasons mentioned above. It's with a little extra wattage. If growing photoperiods and you work backward from needs in flower, you should hang light at the same distance that gives most even coverage and highest average PPFD across canopy, but run it at 66% power for 18 hours per day. Then, you simply amp up to 100% over 12 hours and maintain same exact DLI as well as same evenness of light across canopy.