So your light produces 1890 total? and at 70% currently... that's ~1300 PPF at 70%. If the light is 720 watts and 2.7 umol/J, then the total output should be 1944. so, they either mistyped or are lying on their spec sheet. 1890 / 720 = 2.6 umol/J efficacy. Small difference. Still upper-middle efficacy. 2.8-2.9 are the higher-end. Only the higher cct hit thos 3.1 numbers and hopefully your CCT is closer to 3000-3500K.
Assuming normal hanging distance for LED, how much space is it focused on? in meters-squared.
1300 / area in meters squared = PPFD. PPF / m^2 = PPFD .. now, some is absorbed by walls and some misses the plant, so this is a slightly exaggerated number, for what it is worth, but relative to normal hanging distances, ~40 dli is a good target.. if it's techinically ~35dli at the canopy, so be it, but even though the calculation is obvioulsy precise, the application is looser. Local variables impact the dli you can provide, so there's no single target to shoot for unless you can account for all that stuff locally.
Reference PPFD with hours of operation on a DLI table or caclulate it yourself.... is it drasticlly higher or lower than 40 dli? Based on total output at diode (~1300), this generally works out well to shoot for something near 40 dli.. .you may still have to fine-tune this based on growth that you see.. node spacing of new growth is the guide.
over 12 hours and a target of 900ppfd, you'd want ~85 umol/s per sq ft (if you prefer imperial) Just dividing 900 / 10.764 -- think 900 ppfd over 12 results in nearly a 39 DLI .. you can see the exact ppfd is not necesary for this first step.
600 ppfd is a good target for 18h. Also, ~39DLI.
if the light is properly sized for the area, it's 67% over 18h in vege and 100% over 12 h in flower... all other factors the same, it's inversely proportional to hours of operation. 2/3rds the hours is 3/2 the power to maintain same DLI (assumes same light or same exact efficacy lights)
temp, rh and ambient atmospheric co2 all equate to a max DLI before damage is caused or light inhibition behaviours kick in. So, "max" is variable. You will always have to adjust for your local variables. What someone else does or this math above is only a good ballpark to start in.
anyway, work this stuff out in vege phase when you can assess node spacing, and unlike above, a calculation about what you learn based on growth over 18h will translate very precisely to 12 hour flower operation. Compared to 18h, need 150% more power... hopefully vege is not greater than 67% or the light is too small for the area, if growing photoperiods.
this will also help answer your question about the slight coloration of the pistil tips ... hopefully the light is not related. hopefully, it's wind and not pollen, too.