Solved! Go to Solution.
What I think you want is to convert the true north value to a new field with a grid based bearing.
If this is the problem you are in luck - there is a command for that! Not sure when it appeared 10.0(?)
That refers to an earlier obsolete tool Calculate Geodesic Angle if you are still on 9.x
CalculateGridConvergenceAngle_cartography (in_features, angle_field, {rotation_method}, {coordinate_sys_field})