Research and google it. There are many opinions out there.
I personally top at the 4th node for a Scrog. My setup has an adjustable screen height so I start Scrog tucking once the plant is about 5 inches tall.
If you cannot adjust the height of your screen I would put it about 8-12" above the base of the plant. Let it grow through the screen on its own and then tuck once it's a few inches above it.
Others like to LST and top for a few weeks before they even put the plant under the screen.