.....and I guess that's why most systems are usually limited to about 70 degrees North /South for successful alignment:
The circle drawn during the 10 minute alignment process is too small ('short') to accurately calculate the latitude.
And a little note on the magnetic input:
The model is called the 'International Geomagnetic Reference Field and is updated every 5 years. There's good info on all things magnetic:
http://www.ngdc.noaa.gov/seg/potfld/faqgeom.html