A method for modifying an IC layout using a library of pretabulated models, each model containing an environment with a feature, one or more geometries, and a modification to the feature that us calculated to produce a satisfactory feature on a wafer. The model may also contain a simulation of the environment reflecting no processing variations and/or a re-simulation of the environment reflecting one or more processing variations. The model may also contain data describing an electrical characteristic of the environment as a function of one or more process variations and/or describing an adjustment equation that uses geometry coverage percentages of particular areas in the layout to determine an adjustment to the modification. In some embodiments, and upper layer for an upper layer of an IC are modified using information (such as a density map) relating to a lower layout for a lower layer of the IC.