ttystikk, if you are not worried about plant count or have room, let some plants get taller for the upper end of the screen/trellis area. Remove them lower branch's you know you will not use. Bring in younger smaller vegy'ed plants for the lower end of the trellis/screen. Now you give a week or so vegy time to get all trained and flip with the pre expactations of their stretch when you flip to 12/12 light schedule.
If you want, some would feed a 50/50 mix of vegy/bloom during that first week giving the plants more stretch. I know I would just keep on the vegy feeding cycle the first week when doing horizontal screen with indica's to fill in more, at the same time cleaning all material off the plant below the screen. Drop an hour of light, and nail them with bloom nutrients, yet another way was slowly drop an hour of light from 18 hours as you train that week of vegy feed, switch mix and light schedule when you can anticipate the stretch to fill in the screen.
I used to monitor my daily growth rate to aid me in figuring out when to flip into floral and have a full screen. If I got more than I wanted, just trimmed it back so I only had what the screen could handle.
Remember: many bud sites is not always the answer as more selective bud sites for good bud growth. I found topping a plant at 18 inchs, leaving only 4 selected branchs on the stem, flipping them after a week recovery, double this strains yield for me. The key was I had 4 nice huge colas to trim up, not 200 plus little ones that took days.
I strongly believe knowing your strain and growing it constantly allows one to learn it, predict it, and you can improve your method, feeding schedule, and the overall yield from each grow as you learn more. How did I remember it all from one grow to another, well I kept grow journals on my computer (not the smartest thing) but I did it that way.