1,585 PPF -- on GD's page for this light. Good for them for using the proper measurement. Many companies would call that ppfd or worse, lol.
Full power or 'vege' power if growing photoperiods... Hanging distance when light is properly sized for area of coverage should be about a good spread of light across canopy without sacrificing overall average of entire canopy. A light meter can help... What is 'best' is a bit of a grey area due to variables that impact 'max' DLI you can provide. Also, maybe a little sacrifice of overall average to improve edges or sides a bit might be worth it? I wouldn't assume, but either way will be in the vicinity of what is best without proper testing various options in this regard.
It's a quantum board, so probably better 20-24" away to improve spread of light.
Good starting point for power:
600 ppf per m^2 for 18h operation
900 ppf per m^2 for 12h operation
2 lights = 3170 ppf
32 sq ft converted to 2.97m^2, call it 3m^2 for fun. So, 1800 ppf and 2700 ppf are the targets, respectively.
1800 / 3170 = 56.8% power -- just a ball park. Whether you do 50-55-60% for 18h operation to start, you still need to observe plant and make and adjustments based on node spacing. How the plant grows will dictate the exact power you want to use.
2700 / 3170 = 85%
So, these lights give more than you need for 32 sq ft. That's good. A little extra means you can ramp them up over the years to mitigate any dimming that occurs. By 30-40k hours, it'll be 90% of original intensity. I can tell by the 2.5umol/J efficacy that they do not adhere to samsung.com testing paramaters, so the L90 (when it hits 90% of original intensity) will be faster than the common 50k hours advertised longevity - bsed on L90 rating.
If you measure a specific distance from center under the light and compare over the years, you'll see that value slowly fall... this can tell you when to add a bit more juice as time goes by. Klux is 1:1 proportional, so a 10% drop in klux is a 10% drop in photon output of that light. This sort of tool is also handy when deciding on hanging distance. How much of a proportional difference do you see from center readingsn to corners/sides? That is the percent-difference in PPF too, even if you don't know that value or don't have a quantum meter. Don't pay 500 usd for a quantum meter, lol. You can shave down the learning curve just as easily with a free app and a little math comprehension.
knowing the exact PPF of any single location measurement doesn't really matter. You don't know the target "maximum" without trial and error because Temp, RH and ambient CO2 all impact what that number is... it is a floating value based on those variables. Observe and adjust is necessary regardless of whichever advice you take. There is no one-size fits all answer.
Less than full power --
Can reduce power significantly, reduce distance from plants and cover a much smaller area, if needed for seedlings and small vege plants. Better to buy a smaller light to handle that stuff as the size of the frame is a bit of a limit here as far as saving watts covering a much smaller footprint of plants. Probably won't be using this light with seedlings... but again, here is where a lux metere can help, if you have hammered out how to use it for the full power vege or flower contexts above. If you want to give a seedling 50% of that, you'll want the power and distance to refelct a 50% klux reading.
i find seedlings want near-max light within 48 hours. If i don't I get a shit-ton of stretch, so not sure how people get away with much lower values than that... again, no matter what the suggestion, the reatilty of the plant infront of you is the most important thing to pay attention to. Growth pattern will tell you if it's happy with the light intensity, and that happiness will automatically consider all the various factors involved - temp, rh, co2. If you get a healthy node spacing, you know it's as much as the plant can handle without damage.
How much the plant can handle can vary by seasons, even indoors. Less ideal VPD in winter will result in a much lower max DLI that you can provide without damge, e.g. The more tightly you control your climate, the more consistent this will be throughout the year.