Yea, 1.05... oops again. Never trust what I say!
If you look at the PPFD -vs- phototropic efficiency chart, anything above 700 starts to look similar but the response change is still a curve the whole way. I tried to pull the curve out of a single 3 point reading but it could still be worthwhile to consider the original data.
I used the 30C data for the graph, which seems like the best metric to me. I have also averaged the top three readings and the shape is much more of a hump, less gains as a percentage from the 1000-1500 mark. But perfect temps is the best way to take advantage of the ideal PPFD whatever a person deems that to be. If the goal is to grow as much as possible in a 2x4 the best PPFD would be higher than a goal of using the least electricity without seeing GPW suffer much. GPW might drop off from a peak on the low end, but this cannot be determined by the graph. What is the perfect PPFD as far as electrical efficiency? It depends on whether yield per energy unit increases at ever lower intensities.
If you put a CXB3590 at 1.75 amps over a 4x4 you have a PPFD of about 110. Would that grow 1.5-1.6 gpw (similar to higher intensities with same electrical efficiency), or 1.7-2 GPW or more? I don't know. Somebody should test that
I'm working on figuring that out using less extreme methods but it's still a work in progress.
Some have said the sweet spot is between 800-900 but there has to be an inherent practical/subjective reason for it, space concerns and wanting a "bountiful" harvest mostly. I've played around with 730 and 770, but at different efficiencies. At 730 PPFD I have achieved 3.43 gppw (grams per par watt). At 770 PPFD I have achieved 3.21 gppw. This data indicates the best GPW will be at or below 730. More data would be useful to see if it was a solid trend and whether the trend began to break down at some lower PPFD.