(Edited: Note that Polaris is used in the diagram to indicate 'true north', and ancient versions would need to use different reference points depending on epoch, due to precession)
From the book:
One day in 1874 a Reverend M. Gass, assisted by two students, was engaged in opening a small burial mound near Davenport, Iowa. Near the surface they found an intrusive Indian burial of obviously modern date but, as they descended deeper into the mound they began to uncover the skeletons of the persons for whom the mound had initially been raised. There were two adult skeletons, and a third skeleton of a child placed between them. Nearby they found an engraved tablet, now known as the Davenport Calendar Stele.
This tablet carries an engraving consisting of a central scene surrounded by inscriptions in Egyptian, Iberian Punic and Libyan. The Iberian and Libyan texts both say that the outermost hieroglyphs contain the secret of how to regulate the calendar. The hieroglyphs themselves say (literally):
To a pillar attach a mirror so that at New Year the sun being in conjunction with the Ram in its house at the tilting of the balance in the Spring the Festival of celebration of the First of the Year and religious rites of the New Year are to take place when The Watcher stone at sunrise is illuminated by the sun. Star-watcher Priest of Osiris of the Libyan region.
The reference to a mirror is the key that I'd never previously encountered in any discussion about calendrical markers - but its use is obvious when you think about it, and the resulting geometry of the marker system is trivial.
This older article provides my textual description of the method used to set up the system.