ok...now once you get Convergency for a particular lat, you can find the gc track. Along the same latitude, r/l track from A to B and B to A is 90 and 270 respectively. And now that you have C, you can get CA and hence GC track A to B and B to A for any particular lat. That way, you can find the mean lat and hence the difference.
Note : the Gc track will curve in case of same lat in the direction of the hemisphere unlike r/l which curve in the direction opposite to the Hemisphere of the lat. Will prove to be important coz different direction means higher/lower gc tracks.
Hopefully I didnt embarrass myself again giving solution for distances rather than GC
