I like to (mouse repeater on) broadcast my mouse when setting up the Grid position.
A lot of addons can have their position edited via a file in the WTF folder too.
You want them to be as near as exactly the same as you can manage.

When you make the repeater region, there's the Sync function, so it should be the same on every screen.

It is more a question of positioning the addon to be the same, then of the repeater region.