depends on how you use them...
I use my first scrog layer 4-5" above rim of my pots. It's to help train it out horizontally until the branches get into position.
The second scrog is for support. How high you hang it depends totally on expected growth after flip. It is not needed for most plants, but you come across a lanky, floppy pheno, occassionally, and best to be ready for it.
My diary shows a weekly progression from the point it was under the scrog. You can see i hung my second scrog too high, but it'll work for the 1 lanky plant i have.
The scrogs are also useful structure to shape the plant. Just added some garden wire to spread out a plant. easy to attach and tug the branches outward from a cluster.