Spence speculates that Egypt's pyramid builders found true north by using a plumb line: when the stars Kochab and Mizar were seen on the same vertical, one was facing north. As evidence in support of this hypothesis, she points to the proposed interstar-line precession past the north celestial pole at a rate of 27' per century (cy). We argue that a mathematical error affects this result, which when corrected points more strongly to a different pair of stars. This suggests that the conventional ancient chronology, instead of being compressed, may actually have to be expanded slightly.