chain watcher correct position

What is the correct or best position to install a chain watcher, and is
there any way to position it to guarantee that the chain will be
repositioned correctly if thrown past the inner chainring when changing
from outer to inner chainring.

Thanks & regards

i'm assuming you're talking about the 3rd eye chainwatcher: i set mine
up so that the bottom edge of the chain-watcher is even w/ the
bottom-edge of the chain on the small cookie, and positioned so that it
clears the chain by ~1mm.