What I like to do is, leave a comfortable amount of room under the screen, enough to get your hands under the screen to move and manipulate your plant, ( hence the 8 to 12 inches), reference, but ive had my screen as high as 20 inches. I like to use bailing wire to make my own screen, I leave 2x3 inch rectangles. This gives me the ability to move the wires over when i have to, to bring the stems up into different areas. You got it right, if you want to increase yield, scrog is defiantly the way to go if your using a 400 watt hps or cfls. You can then get your whole plant under the light scorce, not just the top. Now, I have seen folks use the chicken wire and do pretty well, but making a frame the size of your grow area, and weaving wire or twine is soooo much better. The idea of the screen is to be able to open your plant up. With chicken wire, you are not going to be in any position to pull the buds down back through and move them over as your plant grows, ( as the plant is growing in flower, I may chance a buds position in the screen 3 or 4 times). you kind of defeating the ability to do this. The previous post about fimming or topping is correct, you want to get that plant bushy, and push a budsite up through each 2x3 opening to make an even canopy of colas. After the first week of flower, I start removing the foliage under the screen, I take my first round of clones from here. I take all the popcorn bud that are growing directly from the stalk between this time and my final removal of foliage, which I do at day 30 of flower. At this time I get any remaining clone s I may want.