From my point of view the windings will be charging up all the time the pole is under the magnet and will stop charging virtually the instant the pole emerges from underneath, the spark is caused by the rapid collapse of the field when the secondary circuit is open circuited by the points.
This is when you will get maximum spark, any further and the voltage in the winding will start decaying and therefore the back EMF produced will be less.
In a dynamo or alternator if the armature has been skimmed the resultant larger air gap considerably lessens the units effectiveness so I would assume a similar thing applies to when the armature pole comes from under the magnet in a magneto, ie more than a few thou past and voltage decay sets in
This is when you will get maximum spark, any further and the voltage in the winding will start decaying and therefore the back EMF produced will be less.
In a dynamo or alternator if the armature has been skimmed the resultant larger air gap considerably lessens the units effectiveness so I would assume a similar thing applies to when the armature pole comes from under the magnet in a magneto, ie more than a few thou past and voltage decay sets in