The yield really depends on how much light you are using, and how well spread out that light is over the screen area. I'd aim for 5000-7000 lumens per square foot as long as the temps are in range.
Check out my last two grow logs in my sig, they both have examples of Scrogs.
In short, you want to train your plants with LST from a young age. Topping can also help, but I don't like using it as it increases recovery time (and total veg time). I put my screen as low as I can while still being able to water the soil. Once the plants reach that level, put the screen on, and every few days pull the growth shoots under the screen and out towards the edges of the screen. Sometimes I'll use twist ties to pull a node up to the screen so that more of the branch is pressed against the screen, and all the nodes above the twist tie get more light.
With my Trainwreck, I filled up the screen about 60-70% before flipping to 12/12. With a sativa dominant strain, you might only need to fill in 30-40%. After flipping, continue pulling the shoots under and out to fill the screen. Once the screen is filled (or the plants stop stretching), let them grow up and through. I'll snip the tiny little bud sites below the screen, and if there are a lot of leaves laying on top of one another, I'd cut them too to prevent mold.
15" x 20" screen (about 95% filled) with one plant, using 400w hps = 101 grams dried (third grow)
Did a little better with two plants (second grow) and a larger screen, but messed up on the nutes and lost probably 20-30% yield. Keep in mind also that the TW is not a high yielder.
Here is where I learned most of my info before starting:
http://boards.cannabis.com/closet-ca...ly-ladies.html
Happy scrogin :jointsmile: