A method for generating a ferromagnetic shim calibration file includes the steps of performing a computation of an analytical Finite Element solution of an internal magnetic field for each location where a shim is assumed to be present in an imaging volume of a magnet, performing a computation of an equivalent magnetic dipole moment of a shim with a saturated magnetization using the analytical Finite Element solution of the internal magnetic field, making a correction of the computation of the equivalent magnetic dipole moment of the shim to compensate for an aspect ratio for the shape of the shim, making another correction of the corrected computation of the equivalent magnetic dipole moment of the shim to compensate for a magnetic mirror effect in a pole-face of the magnet, and storing all computations and corrections for each shim in a shim calibration file.