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I shoot for plus 4 to 6 seconds a day in a movement that is running well, meaning little variation from one position to another. The reason is somewhat arbitrary; watches are meant to get people somewhere on time, and most people have a better tolerance for being a minute or two early rather than a minute or two late.
For most watches I do three positions, dial up, crown down, and upside down (the 12 o'clock marker down, 6 o'clock up, crown on the left). The first two positions are usually pretty close to each other, and give a good 'rough idea' how sucessful the service was. Why these three positions? Dunno. They seem to give me the best results with the least investment in time and trouble. Other watchmakers may do it differently.

Railroad pocket watches are done in six positions because of the multiplicity of adjustments that can be made, and usually can be made to be very consistant.

COSC calibrations are done in five positions, three temperatures, and fifteen days, and must meet a strict set of criteria. They're beyond my meager talents, but are worth researching if you are interested in such things.

Yes, a watch that is running +4.7 seconds a day could be tweaked to be +-0, but that will only be at one temperature, position, and power condition. Also remember that arm movement will also effect the balance wheel amplitude silghtly every time you move, so it's impossible to correlate machine based regulation with every-day use with 100% certainty.

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