I am sure you already have thought of this Tom but is it possible that someone in the past fitted an almost correct balance staff? Its not just the pivot length and end shake you have to think about, if the main body of the balance staff is to short, just extending the pivots/pivot could move everything the wrong way i.e. either up or down a fraction depending on which pivot you extend. Hope that makes sense? If you use Fried’s method it will help you determine what the measurements should be. There is nothing to say the last owner didnt pick up a rough balance staff and just cut it a bit so it almost fit.
Paul.
p.s. deleted duplicate 
