Well, SH3 (and SH4) hold the rule that 1 degree is 120 kilometer. In both longitude and latitude. So by that rule the circumference is 43200 km. You can measure that with the line tool on the map. The markings on the edges of the map indicate where the exact number roll over. Then by the circumference rule, dividing by 2*pi the radius becomes 6875.49.... kilometer.

And you can prove that by taking any easily distinguishable point on the SH3 map, measure the distances to the equator and prime meridian, divide by 120km and you get the exact place in decimal longitude and latitude that you can verify in Google Earth, or whatever deeply zoomed in map.

The mission editor gives exact coordinates of the cursor. So that is easier to find the coordinate.

Initial starting position of U-boat in SH3-mission "Naval Acadamy / Navigation" is
Long=1165300.00
Lat =6632750.00
Aro Sund
https:////img.xrmb2.net/images/206990.jpeg


To convert to real lat/long:
E 009° 43' 15.0'' = 09° 43.268' = 09.721133333333333° = 1165300 / 120000

N 055° 16' 22.5'' = 55° 16.375' = 55.272916666666667° = 6632750 / 120000