They usually determine this from changing spectrums of red light from my understanding and it's been a while since I read about this so I could be wrong.I believe that the plants like consistency. I did what you're doing and ended up with hermies. There isn't a lumen meter that the plant needs to fill per day, it literally needs light in time not necessarily strength. The strength creates denser buds etc but you can flower under a 100 watt Aerogarden (been there, did that. Smoked the whole harvest in a weekend )
I also feel your pain as far as your set up. I grow 25 legal plants in the 2nd bedroom of my 8th floor apartment. I have a flowering tent and a vegging tent and another set up for herbs, seedlings, and now some wee cuttings.
There's also evidence that plants can do math (calculus) whereby they predict how long the dark will be and use their stored energy appropriately & accordingly. The math comes in so that they're not "starving" for the last few hours of darkness. This is why consistency is good for them where lights on hours are concerned.
Plants Do Math to Survive the Night
I agree with you on consistency. The argument on the 6 on 2 off is about how efficient the plant processes are over periods of time. Some evidence shows under high lighting that the plant processes for photosynthesis slow down and halt once light saturation happens and photorespiration then occurs slowing growth. The idea is that doing a 6/2 cycle would eliminate that. I can say I see no reason to assume that plants have a 24 he clock and feel they will adapt to consistent parameters within reason