I doubt it's drifting - at least you have not indicated as such. as the thread I commented in on WUS shows, this is set by the watchmaker at service and can be adjusted then. Once set it should stay set.
When was the last time the watch was serviced? you mention good amplitude above 200, but you don't say if the lift angle on the timing machine was set correctly (often they are not - should be 49 degrees for this movement) and what the position or state of wind was when that reading was taken. If that is full wind dial up or down, then 200 degrees is not good amplitude...
Cheers, Al
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