I think the tendency is to make it more technical than it is. Using varying lengths of movement on the stones, you get a taper.
As I noted previously, line markings on the rod is what I used. When you look at the entrance taper and different types of
chokes on the market in modern stuff, you see different lengths of taper, different lengths and in US styles, no end-choke which
is what I did with the "too tightly choked" 10 bore, by cutting off the 1 1/2" of end-chokes (distance from the end of the choke to
the end of the tubes and then relieving the chokes until they shot well.
This kinda taught me there is a lot of lee-way in what you do, and patterning while you work on the gun, is necessary. There is no
concrete formula I am aware of. I am sure factories have their measurements, but suspect they will not or do not divulge their secrets.
The factory choke didn't work, but my modification sure did. From weak modified to extra full choke.
68% to 94% and 96%. With the #2 shot, that was something like 9 & 6 pellets out of the 30" circle at 40yards. This was with lead shot.