Very interesting mix of grow techniques, and mediums. I dont think Ive seen anyone else try this combination - hydroton + coco watered via F/D. It sure looks like its working great for you!
I have found it works well to flip to 12/12 a bit before the scrog screen is completely full. Maybe 70-80% depending on how tall the strain wants to grow. The babies will continue to stretch some after you flip, so its good to have a little excess room to put the extra growth. I got away with about 80% full on my mostly indica strains, but I needed to flip with more open space when I did the taller growing sativa type on my last grow.
Edit: Almost forgot - with a dense scrog canopy like yours, it might be a good idea to add another small fan to circulate some air below the canopy. Or, make sure you have an air inlet above and outlet below or the reverse. You dont want it to get too damp under there.