If the a/c travels due west from A (constant track, implies that it is a
rhumb line) it will remain on the same parallel of latitude (45oN).
However, because it is not at the equator, this RL track will not give the
shortest distance, as the equator is the only parallel of latitude which is a
GC.
Therefore, to fly the Great Circle from A>B, initial track must be a
little more than 270, it must be 270 halfway between
and then must be a little less than 270 to finish with. (This is based on the
fact that GC tracks 'curve' convex to the nearest pole.)
The convergency tells you how much the track changes between A and B (6
o) so it is 3o more than 270 at the start and 3o less
at the end (273-267=6 o).