A computer-implemented method is provided for generating an electromagnetic field (EMF) correction boundary layer (BL) model corresponding to a mask, which can include using a computer to perform a method, in which asymmetry factor data is determined from aerial image measurements of a plurality of different gratings representative of features provided on a mask, wherein the aerial image measurements having been made at a plurality of different focus settings. The method may also include determining boundary layer (BL) model parameters of an EMF correction BL model corresponding to the mask by fitting to the asymmetry factor measurements. Alternatively, the asymmetry factor data can be determined from measurements of line widths of photoresist patterns, wherein the photoresist patterns correspond to images cast by a plurality of gratings at a plurality of different defocus distances, and the gratings can be representative of features of a mask.