Hi,
best practice for the marker would be to point at the spot where they expect the pivot. If the thrower disagrees, they should point to a different spot. Whenever the thrower sets their foot (or any other body part) there, you should start stalling.
And now for what the rules say (to be used if above was not done):
the rules require the thrower to approach the pivot spot with at least walking speed. Thus, it is fair to assume that the thrower has set their pivot the moment they contact the playing field *and* have stopped walking. Remember, the fair assumption inherent in the rules is that no one intentionally violates the rules, and that everyone knows the rules. Stopping before the thrower reaches he pivot would be such an intentional violation, which the rules say you do not need to take into consideration during normal play.