You kinda have a choice. Static time it to the F mark and then check it with a strobe and the advance timing will be a little off.
Time it to the strobe and advance marks and the F timing will be a little off.
#2 is my choice. Very unlikely to get both spot on, IIRC.
Has to do with cosmic factors like production tolerances of the points, heel and contacts, point wear, dwell, phases of the moon, etc., advance spring fatigue, 3rd year of a Presidential cycle.
Personally i would not go to work on the advancer.