LED lights

6x4 = 24 sq ft / 10.764 ft^2/m^2 = 2.23m^2 Read wiki on Daily Light Integral to understand the metrics used... PPFD is per meter-squared and not a single point measurement. You can reference pre-made DLI tables with PPFD and hours of operation to find DLI. 35-40DLI is a good max output target for current LED grow lights and their typical hanging distances. Autos / Vege with 18 hour operation 2.23 * 550 PPFD = 1226.5 umol/s PAR 2.23 * 600 PPFD = 1338 umol/s PAR 1200 - 1350 umol/s PAR output from your lights is well-sized. That's 35-39 DLI, rounded. 18 hours is not a hard number. You could do 16/8, but you'd need proportionally more light over 16h compared to 18h. Same DLI = same yield, all other factors remaining the same. It is still important to have a dark cycle. So, more is not always better. The only reason to go beyond 18h is if you have a light not powerful enough for the area of coverage. That might outweigh the costs of whatever neglecting the dark cycle may bring. Something to avoid, not plan for. Photo period 12 hour operation needs in flower phase... can just apply 3/2 conversion from 18h or do the math above a second time with roughly 800-900PPFD. 1800 - 2000 umol/s PAR output for 2.23m^2 is roughly 35-39 DLI provided. Common sense hanging distances that don't sacrifice overall average while reducing difference from central readings to edges/corners as best you can. Quantum boards - 18" give or takes. Bar-framed lights can get a bit closer on average because they have a larger frame to help distribute light better. An overpowered light for the area may need more distance than normal or fewer watts (dimmer knob), dealer's choice. No matter what, all suggestions need fine-tuning. You'll observe distance between new growth nodes and adjust light intensity based on that. What these suggestions do is get you into a very small ballpark around what's 'best' for your local variables. Climate and ambient co2 will impact exactly how much light you can give. It is not a static number. It is relative.