1. A method for modeling critical-dimension (CD) scanning-electron-microscopy (CD-SEM) extraction during photolithography process model calibration, the method comprising: receiving measured CD values which were obtained using a CD-SEM extraction process, wherein the CD-SEM extraction process determines a measured CD value for a feature by measuring multiple CD values of the feature along multiple electron beam scans; determining simulated CD values, wherein a simulated CD value is determined based at least on a set of CD extraction cut-lines; and during photolithography process model calibration, fitting a parameter that models an aspect of the photolithography process based at least on both the measured CD values and the simulated CD values.