I roughed it out and then fitted it using the wittness marks as a guide, but I had the angle a little wrong and the fit never took the good firm feel.
I then blued the socket with bearing blue and found only small areas of good contact, I was able to fit it better with the bearing blue as a guide, but when I bottomed out on the step to the butt I was very close but not fully bearing.
I'll cut the shoulder back 4mm, as 2mm should get me seated very firmly bearing fully, leaving a 2mm gap to the shoulder. This should only shorten the butt by 2-3mm.
Note to self, always use bearing blue from the start with this sort of thing.