Boots507:
Stopping exactly on the far lip is required in order to properly reverse to the new layer and keep a tight packing without overlaps.
By measuring the wire thickness you could estimate the number of turns needed but I don't see how you'd ever get it exact.
I don't know whether it would be practical with wire that fine, but I wonder whether it might be more effective to detect the outer diameter change as the wire hits the lip at the end of the former, rather than try to predict when it will hit? If you used a floating pulley, you'd need to detect a movement equal to the thickness of your wire.