Well the idea is you figure how tall you can make your plants at the end and work back from there. You need at least 30 day's for the plant to develope all the antioxidents in proper portuions for a good flower. (If you are growing from clones then don't worry about that). So lets say that they are 24" tall at 30 days. They will end up about 38-46" when finished so you place the screen at 30".