Consider the triangle made up of:

 

A: 61°30'N on first meridian

B: 58°30'N on second meridian

C: 58°30'N on first meridian

 

The track AB at A is 120°T so the angle in the triangle at A = 180° - 120° = 60°

 

The angle in the triangle at C = 90°

 

Side AC = 3° latitude = 180nm

 

AB = hypotenuse (h) and AC = adjacent (a) to angle A

 

CosA = a/h therefore h = a/cosA = 180nm/cos60° = 360nm = 667.3km

 

Scale = CD/ED = CD/667.3km = 1/1,000,000

 

Therefore CD = 667.3km/1,000,000 = 66,730,000cm/1,000,000 = 66.73cm