MA: It could just as well mark the end point. We don't know.
SC: Since the start of anything comes before the end, it is likely they would have marked the beginning of the precessional half-cycle (as the commencement point) and not the end. This is to say that if precession was moving from min to max (G3 to G1) then we would have found a corner of G3 touching the circle and not G1. You don't fire the starting pistol when the runners are already over the finish line. Also, it seems the builders used the figure of 5600 (i.e. the number of years that had passed from the previous max culmination) to the timeline intersection point as, as they seem to have multiplied this figure by the pi approximation of 1.3142857142857143 to give the other side of the timeline of 7360 years thus making the classic precessional or Platonic Great Year (half cycle) of 12,960 years.
So it does seem that they began with the previous Max culmination.