The more dialed in your grow is the harder you can push the light. I run a little over 300w in my 3x3....a SF se5000 at 60% or a little over. The nice thing about an overpowered light is you can run it at less than 100% to extend it's life.
And you can raise the light to 18" or more and still get 1kµmol on your canopy and still keep the temperature at the canopy level <78°.
That's the whole trick - getting lots of light on the canopy while keeping temperature of the tops of the buds <78°.
Almost any modern grow light will get 1kµmol onto a canopy. Some lights will be at 8" hang height and have an uneven PPFD maps, but you can get a lot of light on a canopy for cheap.
The trick is to get an even PPFD map + have to PPF to be able to get the light bars high enough above the canopy to keep temps down.
Per Mitch Westmoreland, below, once the flower tops hit 78° (or less, depending on the strain), cannabanoids are lost. I grow at high light levels (>1100 for photos and 80±mols for autos) and have had some very high yields but I let my temps stay in the low 80's in flower. The crops have been superb but the kick wasn't there. The culprit, I firmly believe, was that the temps were too high.
So, don't turn the PPFD down (unless your grow can't handle it and, if that happens, fix your grow) but crank up the wattage and the hang height in flower.
Just took light readings for my two Gorilla Glue plants in my 2' x 4' tent. Hang height is about 10". Temps at canopy range from 72-75.
The highlighted values are left front, center front, right front readings. I was interested to document the difference in PPFD when the flaps on the tent were open vs closed. The other readings vary because of the inherent error in reading PPFD manually but the ∆ of almost 100µmol (on average) is noteworthy.
The other issue - cannabis loves light. The more light you give it, the more food it generates for itself, and the bigger it will grow with no change in potency (up to the light saturation point).
