The largest circle is the default dispersion of a given cannon, the ball may land anywhere inside circle. There may be some percentage chance it lands closer to the center "aimpoint" than towards the edges, but we know nothing if they have such calculations going. Besides, we don't aim our guns like a normal FPS crosshair shooter anyway.
So with the Congreve sight we can tighten this circle at the end of the firing cone by 5%. There are no multiple calculations; the trajectory of any given ball is given at the moment of firing.
This is an old pre EA connie firing with 10% dispersion upgrade:
jodgi's post in CANNON_DISPERSION_VERTICAL was marked as the answer