1. A method comprising: determining a first flare convolution based on a feature density of projected structures on a substrate layout, calculated on a coarse grid over a unit area; determining a second flare convolution based on a feature density of projected structures on a substrate layout, calculated on a fine grid over the unit area; determining a system flare variation over a unit area of a substrate layout summing the first flare convolution and the second flare convolution; determining a critical dimension variation based on the system flare variation; and modifying a feature density layout to modify the critical dimension variations.