Aspects of the invention include a computer-implemented method of designing a photomask. In one embodiment, the method comprises: simulating a first photomask patterning process using a first photomask design to create simulated contours; comparing the simulated contours to a desired design; identifying regions not common to the simulated contours and the desired design; creating desired target shapes for a second photomask patterning process subsequent to the first photomask patterning process based upon the identified regions; and providing the desired target shapes for forming of a second photomask design based upon the desired target shapes.