No wonder we stuffed up vietnam in the 70's
with such hocus pocus the army gives it's solders
the rate of magnetic deviation is not a fixed variable! and is different in all locations based on the geology of the area.
i.e. magnetic mountains will throw a compas out and even make it spin!
Ring your closest local airport they will give you the current variance
i.e. 11° E (01/04) Toowoomba
http://www.airports-worldwide.com/australia.html 