I feel silly mistaking the question. I spent a few hours preparing some illustrations of my hypothesis about how the cut angles on classical corners.
Since they're made, I'll just post the illustrations.
To work the cut line for the corners, you mostly need to know the bout line, the center for the main riser arc, and the corner circle centers. The first illustrations shows these. The second shows there relationship to other elements in the design.
You then drop arcs from the outer corner circle centers, using the riser centers. The intersection of these arcs with the bout lines are what we want to find.
The corner cut lines either pass through this intersection, or through a simple division of the line segment from this intersection to the edge. For most classical examples, this hypothesized method yields a good match to the real corners when the upper corner cut lines run from the intersections, and the lower corner cut lines from the 1/2 mark. However, to allowing using the actual intersection, 1/2 way toward the edge, or 1/4 way to the edge for either upper or lower bouts. If we allow this, then the method yields good matches in all classical examples so far examined.
In most classical examples, including the 1645c N Amati used here, there is significant wear or perhaps original rounding in the corner shape. So this makes the angles somewhat ambiguous.
Here is another example where the cut angles is much clearer. The instrument is the 1680c Ruggieri 'Milanolo' violin.