Funny, my first thought is "I want to measure this"
$$$ finding excuses, yeah right.
I like his metric, DLI target set to 90% of the overall PSII efficiency curve. So, maybe determine the PPF required to achieve the DLI needed for both scenarios, veg and flower. Then, perhaps slowly ramp up the intensity coming out of veg to meet the target DLI when switching into flower? Or, maybe the other way around, determine the 90% for flower with reduced intensity in veg. idk. Of course, I'd think that the PSII efficiency could be optimized to some extent with the environment, e.g. CO2 to ensure easy access to carbon for fixation.
There was also some interesting research on the effect to the PSII efficiencies when we enter into photoinhibition or damage the leaf from overly high intensities, and how/if the leaf can recover. All of which can be measured, apparently.