billybob,
I am a first time grower, and first time ScrOGer, so I am being dragged along by the seat of my pants. My circumstances are a little bit tough, as I have two little indica/sativa hybrids and a pretty much full sativa. To make things more complicated, the Sativa had about two months of veg, and the little ones only had a month. To make things work, I dedicated half of the screen to the Sativa, and half to the two Great White Sharks. I have the GWS elevated on books.
I topped all of them when they hit the screen, so they started pushing up all of those lower budsites. I have been trying not to weave very much, as it seems to be more effective for me to keep them under the screen. When they grow too much, I just move everything over a little bit so that no bud takes leaves another in the shadows. I did, however, probably go to 12/12 too soon. I became very paranoid about the stretch, and didn't want over growth.
As far as how much to fill before switching, I would say that it depends greatly on what straing you're growing. Do you have experience with it? Do you know how much it will stretch? Can you find out? If it is a stretchy strain, you should go to 12/12 very early. If it isn't, give them plenty of time to fill up. Training isn't hard, and really isn't nearly as labor intensive as I had thought it would be. Predicting the stretch seems to be the difficult part. I am still praying for more growth, but still have about 35 budsites right at the canopy to fall back on.
I hope it all turns out well, and happy ScrOGing! Check out my grow journal if you have time; I have been flowering for a little over two weeks, and they are starting to look gorgeous. I am going to update it very soon, as I have done a lot of shifting around since the last one, so keep an eye out. And if in the next couple of weeks, it looks like it didn't work out for me, ignore all of my advice.
S-Town