light cycle spider sf 1000

For a 2x2 space, 18 hours is all you want... posibly up to 20hours at most. If the specs are accurate, you are up near max DLI with typicaly ambient CO2. i wouldn't go 12" height. I'd got a bit higher for more even blanket and no 1033umo/s hotspot (the spec sheet erroneously lists this as umol/s/m^2.. that's not true unless it is 1023 at every point within m^2 or averages out to that, which it will not.) 16-18" is probably good for this QB and 2x2 coverage. 100w X 2.8umol/J is ~300umol/s. 4ft^2 is roughly .37m^2. 280 / .37 = 756. 800-900 is likely as high as you can go without co2 added to local atmosphere. So, possibly 20 hours will get you to ~850 PPFD. If you see a lot of droop the last few hours, shorten it up. If they keep praying, increase it 30mins at a time and see what happens. Give it time to acclimate.. if you see it drooping a ton in a few days... drop back 30mins. let plant make ultimate decision, but use math to get into ballpark.