@hlangeveld @jesterchen I wrote this yesterday and deleted it coz I felt I've hijacked the OP. But for completeness...

Equator to pole is very close to 10,000km (it was defined as that but an error crept in).

Similarly, a nautical mile is one minute of (latitudinal?) arc, so 90 degrees x 60 mins = 5,400 nm. Again errors occurred.

So km -> nm is roughly 0.55 (11/20ths), so the reverse is approximately 20/11 (~1.818...), which is close to (2-0.2) for easy calcs.