Sonex applogized that my cap did not have a timing mark on it; it should have been there.
Kerry told me to just make my own mark with the edge of a file, roughly centered on the round magnet on the side of the cap. He said that exactly where it goes is not important; it's just a starting point for setting the timing.
So I cut a groove in the edge with a triangular file, and put a Sharpie black mark on the face, as well.