Hey Growmie,
12-14 inches . You want plant to grow outwards and fill the screen, so growing it tall has no benefit.
Hopefully I answered your question and remember Happy Growing and Free your mind one puff at a time.
There's no set answer to this, but at least let enough space so you can water and work under the canopy. At the very least leave 8 inches, 12 would be better.
A real scrog grow you want to put the net as soon as possible and start filling it from early on, and then just lollipop the bottom branches to get all energy to the tops, and get filliiiiiing growmie
Good luck, i'm gonna be watching.