1. A method for a lithographic process for imaging a portion of a design layout onto a substrate using a lithographic imaging apparatus, the lithographic process having a plurality of design variables, at least some of the design variables comprising adjustable components of projection optics used to adjust shapes of one or both of a wavefront and an intensity distribution of an irradiation beam of the lithographic imaging apparatus, the method being implemented by a computer and comprising: calculating, by the computer, a gradient of each of a plurality of evaluation points or patterns in the portion of the design layout with respect to at least one of the design variables, wherein the at least one design variable includes at least one of the adjustable components of the projection optics; selecting a subset of evaluation points or patterns from the plurality of evaluation points or patterns based on the gradient; and adjusting one or more of an illumination source of the lithographic imaging apparatus, the design layout and the projection optics of the lithographic imaging apparatus by measuring a simulated image at the selected subset of evaluation points.