what ppfd for 20/4 autoflower?

35-40 DLI for ambient co2... Simply reference a DLI table with PPFD and hours of operation. 1200-1300ppm CO2 and tightly controled environment, maybe, 55 DLI is a good starting target? The fc4800 puts out 1300 some umol/s. Divide that by area in meters-sqared of your tent -- this assumes it's hung at a distance that creates teh most even light across canopy, but also doesn't sacrifice overall average intensity at canopy. this is your ppfd. e.g. a 4x4 is about 1.5m^2, so 1300 / 1.5 = about 850 PPFD (roudning for easier reference on atable)... this is 61.2 DLI and if the plants can handel that they merely adapted to it as if they would to 2000PPFD you find outside in the sun, but it cannot use that much in a day... it merely adapts pigmentation to accept what it can without damaging itself (i.e. adaptation). Maybe, with elevated co2, this would be useful. No matter what, these values are ballpark ideas to start with, then you have a shorter adjustment period observing and reacting to the plant. if you blast the shit out of a plant early on, it might adapt to it.. some may get stunted.. inconsistency of results will be the hallmark of this. It doesn't matter if it is an autoflower or a photoperiod... 35-40 dli is going to be near your ceiling in ambient CO2. If you know 100% for sure all of the genetics in the plant came from a northern region or a place with really intense light, you may find variation in what a plant can handle. This is rare.. it's all been interbred at this point and jumbled up... just becauase it has ancestry somehwere, doesn't mean it still has all of those traits after being interbred.. in fact it won't have 'some' for sure - either lost or just no longer a displayed trait andn buried due to combination of alleles that resulted for that trait in the 'next ' generation of offspring. autoflower need 'less' light per hour if you run the lights longer in the same way photoperiods do. This costs roughly the same wattage, though, to reach same DLI. The only benefit to the longer hours is you create less heat... in some cases you may want more heat from your lights and in those contexts you should run your autos with fewer hours of light. 2 birds one stone type idea... When i use my FC4800 in vege.. it's about 18" away and 70-75% power but only on a 16/8 cycle. If i were on a more normal 18/6, i'd be clsoer to 67% power. this is a well made light for a 4x4. In flower on a 12/12 schedue, i use 100% power at 18" away. You can see how hours, ppfd are 1:1 proporitonal (inversely with hours of operation) in their effect. if you have 2/3rds the hours, you need 150% of the power to keep same DLI, all other factors the same. so anytime you figure out the best use of that light, as long as you keep same hanging distance, it's just a matter of a little math to deduce what you need with different hours of operation or slightly different PPFD (while adjusting the other to compensate). it's not magic.. it is ruled and ordered with consistent results.