1. A method of calibrating a model of a lithographic process, the method comprising the steps of: providing a plurality of test features of a first tone each of said test features having a different feature width each defined by a distance between two adjacent background features of a second tone, wherein a first of said test features has a first resolvable feature width that is known to be resolvable by the lithographic process, and each subsequent one of said plurality of test features comprises a subsequent feature width that is smaller than said first resolvable feature width and different than any other feature width of said plurality of test features; forming a resist image of said plurality of test features according to the lithographic process; identifying from among said plurality of test features in said resist image a first printable feature width in accordance with an acceptable printability criterion and a first unprintable feature width in accordance with an unacceptable printability criterion; determining a printability extremum intensity value associated with said first printable feature width and an unprintability extremum intensity value associated with said first unprintable feature width; and selecting a constant threshold value based on a combination of said extremum intensity values, said constant threshold value for use in a constant threshold model of behavior of said resist in said lithographic process.