This is one of the major problem affect game realism.The further you go north or south the more east-west distance are strentched.This make almost the long distance travel time inaccurate by a huge factor.
One way the navy plot were made is too strentch both north-south and east-wast as you move towards earth poles.This way the bearings getween objects are reserved but the distances can not be directly measured.Like these ones:
The way SH3 simulate the enviroment compromise both.Neither distances nor bearings are accurate.This makes plotting in champain file really difficult to resemble actually historical time the place.
One way I overcome the issue is make a few "jumps" alone the waypoints the compensate the distance strenches,but this is not a very good solution.Still wondering if it is possible to resolve this by modification alone.