An East Mag Var means that the Magnetic North is East of True North.
A West Mag Var means that the Magnetic North is West of True North.
If East is positive:
To convert from a Magnetic reference to a True reference, add the Mag Var to the Magnetic Referenced value.
To convert from a True reference to a Magnetic reference, subtract the Mag Var from the True Referenced value.
Graphically, using a Mag Var of West 10 degrees.
Mag Heading = 100 degrees
True Heading = 90 degrees
Example:
Using above: Mag Var = West 10 (−10)
Mag heading = 100
True heading = 90
To convert from Mag to True:
100 + (−10) = 90
To convert from True to Mag
90 - (−10) = 100
Courtesy of (and created by) Mike Olsen