I'm a newbie, not an old hand, but I've been studying like crazy. So take my advice for what it's worth, which isn't much. Others may know much more than me.
I use Photone. It's just not good enough to be trusted. It gives a very general idea, but isn't reliable enough to use in fine tuning, IMHO, especially if you don't have a meter to calibrate it.
Yeah, you gotta get that photoperiod back to 12/12. If you were running 900+ ppfd, maybe you could get away with 11/13, but you need the daily photons to be up there, and with your lights and on your schedule you're just not getting it. But just 2 weeks into flower, you're not going to be seeing much flowering yet, anyway, mostly stretch and pre-flower.
I haven't used your lights, but the PAR map provided by the manufacturer shows a huge drop-off from center to edge, not uncommon for quantum board lights. You should probably have them hung several inches to a foot apart so you get more even coverage. Doing that, I'd probably hang them at whatever height gives the most even coverage, trying to keep it less than 900 ppfd in the middle and above 600 near the edges. Might take some experimenting. You could hang them lower to get more light in the center of your canopy, but then you'll experience a huge drop-off near the edges. The problem I'd have with this setup is that your lights aren't strong enough that you can hang them a good distance up for even coverage and still get enough ppfd for a good flower yield. I'd frankly invest in a better bar light.
I have basically your same setup with different lights. I opted for a stronger bar-style LED light, for more even coverage. I bought the Grower's Choice ROI-E720, which is a commercial quality light. Yes, it cost me a bunch more money than you spent. But I can hang it high and have very even coverage over the canopy while still maintaining around 900 ppfd in the center and 790 ppfd on the edges. With a 4X4 space and one bar light, with FF Ocean Forest and perlite, in 5 gal bags. I vegged for four weeks and then flowered. I'm in Week 7 of flower now, and the colas are getting big and fat. I'm told I'm going to yield 1.5-2 lbs from this grow (we'll see!), largely due to the light I'm using. IMHO, if you're going to spend money on one thing, it should be the very best quality LED light you can afford. Yours are fine for veg, but you really need more for flower, IMHO. And personally, I prefer bar lights for more even coverage.