A method includes receiving a layout that includes a shape to be formed on a photomask and determining a plurality of target lithographic contours for the shape, wherein the plurality of target lithographic contours includes a first target lithographic contour for a first set of process conditions and a second target lithographic contour for a second set of process conditions, performing a lithographic simulation of the layout to produce a first simulated contour at the first set of process conditions and a second simulated contour at the second set of process conditions, determining a first edge placement error between the first simulated contour and the first target lithographic contour and a second edge placement error between the second simulated contour and the second target lithographic contour, and determining a modification to the layout based on the first edge placement error and the second edge placement error.