14" sounds good. If you are planning on doing a true scrog where you bend and weave the branches through the net as they grow, then I would but the screen during veg. You need to have plenty of space between the plants or you will run out of screen though. What I do when I have the space to do a scrog, I let them pop through the screen and bend them over and actually weave them through the screen. The idea is to have a wide enough screen to keep weaving through the stretch, so you'll have to decide when the best time to put the screen on depending on the space you have. What I do is use the screen to keep my canopy even and support the plants now. It isn't actually a scrog, more of a partial scrog. After my canopy is nice and full, I just let them go. If done right I get a screen of colas and everything below the screen is bare stem. I have had scrog growers tell me that my method is not considered a true scrog, because a true scrog has only buds above the screen without branching. Either way it is a "screen of green" but you will hear a lot of different opinions. The original reason for scrog used to be to spread a plant out when there is limited verticle space, but when growers discovered they could get more yeild per plant by spreading it out to have a flat top instead of a chistmas tree, it has become a widely used method for maximum yeilds. I have seen some amazing outdoor scrogs with up to 8'x8' screens that have amazing yeilds.
I just use the screen to get the even canopy and for support, so I put my screen on after the first week of flower.