You stick it in a collet and eyeball it over the edge or corner, then you look into the viewfinder and see a crosshair. You line up the crosshair with the edge or corner and you're done. Its fast and accurate. Only problem is I don't know who makes it or where to get one (I used one years ago in a different shop).

Anyone know what they're called, who makes them, or where to get one?

Thanks,
Mike