when it takes up about 1/2 the space it needs to fill.... that's how long it needs in vege. Always have to work backward and if not familiar takes a a few tries to feel comfortable about it. some plants will get three times the size... if breeder mentions info on this stuff, good to take note. otherwise start with 2x, and if you know it's heavy sativa, maybe 3x growth expected first time. i figure i can always prune a few select branches off if a bit too large.
70% x 70% = ~50%. so 70% Length x 70% Width of what you want... just a guideline. if you prune things up for shape and any other reason, maybe that is a moot point. in those cases you want to have "enough" colas to fill your space as you want.. 1 cola per 5"x5" slot? usually safe for airflow.