Dale DePriest wrote:

You need
a table of magnetic variations and interprolate from the table based on
your location as there is no mathmatical model that will provide this
information reliably.

Given a table of magnetic variations at known locations, there are
statistical methods to derive a useful mathematical model. The web site
cited in a previous article provides such a model as a polynomial.

http://williams.best.vwh.net/avform.htm#Var