I'm no expert, but my understanding is that PPFD values of around 1,000 are the highest under which typical cannabis can absorb photons. I've read that if you're adding additional CO2 to the grow environment that number could go as high as 1,500. I shoot for 1,000 (which I can get at 24 inches above canopy) and rotate my plants from the center of the light map towards the edges on a daily basis. In my limited experience (based on being lazy and letting the plants get taller without raising the lights!) I felt like I was flirting with light burn on the top leaves with a PPFD of more than 1,000.
hope that helps and good luck with your grow!