Thank you very much for your comments. I finally found by myself a satisfying solution.
The given earth convergency is 20° at N60. The formula dlong = earth conv / sin lat shows a dlong of 23.1° between A and B.
Draw a circle with its centre at the North Pole( NP) and a radius of 1800 NM corresponding to the distance NP to N60, sketch a angle of +/- 11.55°. The secant represents the straight line between A and B. At half way the distance between secant and circumference is 1800 NM x (1-cos 11.55°) = 36.45 NM.
As the given earth convergency is only 20 degrees you now sketch an angle of +/- 10°. The above formula gives the distance between N60 and the great circle: 1800 NM x (1-cos 10°) = 27.35NM
Now you have the following information:
N60 and the straight line differ by 36.45 NM
N60 and the great circle differ by 27.35 NM
The straight line and the great circle differ by only 9.1 NM