1. A method of calibrating a sub-resolution assist feature (SRAF) printing model, said method comprising: generating images of a patterned photoresist layer on a substrate, said patterned photoresist layer having a photoresist top surface that exhibits surface indentation due to presence of SRAFs in a lithographic mask for at least one first combination of focus and dose conditions; calibrating an SRAF printing model against said images by determining a value for an SRAF printing best focus plane and by determining a value for an SRAF printing image plane; classifying said images of said patterned photoresist layer according to severity of said surface indentation and into at least two categories; generating multiple calibrated intermediate SRAF printing models for each of said at least two categories; determining a composite SRAF printing probability estimate based on a weighted average of printing probability estimates from each of said multiple calibrated intermediate SRAF printing models; and applying, employing a machine configured to run an algorithm for said calibrated SRAF printing model on design patterns, said calibrated SRAF printing model to areas in a design pattern within said lithographic mask or another lithographic mask where SRAFs are present for at least one second combination of focus and dose conditions.