All excellent q’s and well worth study and contemplation. All I have been able to learn so far is vague and general like some dark is essential to good plant health. Also hear things like @Capt.Cola mentioned…
I am sure someone would say a specific number but who’s to say that is right for me and my 1000 different environmental issues from yours.
My “seat of my pants” grow:
I am currently using 2 half ass(apparently) lights, a messy home-made dirt/compost/fresh and used potting soil mix, half a tent and miracle grow bloom booster. I just harvested a qp in 10+1/2 weeks from seed. Very low-tech and without all the DLI,VPD charts,Rh and temp regulators.
I should mention that I could give a rat’s ass about award winning results and that extra 5% yield, as long as it is getting the job done.
Although a vote in the BOM contest wouldn’t make me mad…October 2021 Bud of the Month - #46 by AfgVet