Minimum convergence?

I concur with your general point blackdog but it is sufficient to do some basic calculations to understand that passing from 100m to 50m is by no means a drastic change in terms of gun mount.

The following table shows the angle of the gun to point to a convergence to 100m or 50m for three positions on the wing:


ANGLE WRT LATERAL AXIS (90° pointing straight forward):

Distance of gun from sym. plane
Conv. | 2m | 5m | 10m
======================
100m | 89° | 87° | 84°
________________________
50 m | 87° | 83° | 78°

So if one would mount the gun on the wingtip of the 109 (about 5m distance from symmetrical plane) the difference would be a merely 4° while the gun is rather at 2m or even less where the difference is only 2° and below.