I have replaced the hairspring on a #21 balance with one from a #22. Although the diameter of the balance wheels was different, the swap worked and the movement kept time. I assumed that I needed to position the spring on the shaft such that the pin on the balance wheel would be in the same position at rest as it was with the old spring, i.e., in a straight line between the pallet lever pivot and the balance staff(I don't know if this was correct but it made sense at the time.) For future reference, its fairly easy to pry the hairspring off the balance staff and simply rotate it to center the beat.
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