The point is that this is different for every plant and there is no "research" that is going to provide you with an optimal light value like the 400umol you plucked out of a random photosynthesis vs light intensity chart from a high school textbook. That's not for Cannabis, bceause they can use as much as 2000umol/s/m2. They don't even state what plant it's for. It's just made up data to create an example
It's like saying you want to get the optimal nutrients ppm. There is no such thing. Each plant wants something else and during it's life cycle this changes and it also depends on other factors.
For light intensity you can use the photosynthesis sensors I mentioned. You can then measure what is "optimal" for that particular plant at that particular stage in it's development. Or of course go with some global average value. Like the chart we already have for Cannabis. Which shows you get much more yield from 1200umol than from 600.
@MeGaKiLlErMaN, You shouldn't move the lights higher to get lower intensities, but rather dim the driver to get less light. The height of the fixture should be determined by the best compromise between uniformity and wall losses. ie not too low to cause hot spots and dark areas and also not too high to lose too much light on the walls.