You did the best thing by flushing it. Those leaves in your pic seem like they are wilting and wrinkling, thats usually a indicator that your plants ph is off. But you say that you water at a 6.8 so thats probably not the problem. The next thing i would correct is next time you feed it cut all those doses of nutes down in half. You always wanna start off with very small amounts and then gradually increase according to your plants response.
Did your leaf that turned yellow and fell off, did it do it while you was in the smaller pot? Because I have had problems with leaves yellowing and falling off because of being root bound, so its good thing that you transplanted. But yea you are on the right track by flushing, thats the first thing I would have done. Just make sure your ph is good and start low level nutes......OHHHU 1 more thing is your using a 400hps at 27 inches? thats too far away from your plant. You are losing valuable light. I keep my 600 12-14 inches away from my plant. Drop your light closer, my buddy uses a 400 and he keeps his at 8-10 inches away with no heat problems just make sure you have a fan blowing across your bulb