Yes the -10° are correct, at second world war the difference between magnetic and geografic Northpol was around 10° West dependend on Location.
for example 1940-07-03 it was around -10° 23' 36" in London or -9° 3' 52" in Dunkirk.
Here you can find a calculator for this problem:
http://www.ngdc.noaa.gov/geomag-web/#declination