1. A method of measuring parameters of a sample with an optical metrology tool, the method comprising: obtaining an anchor spectrum for one or more reference samples using the optical metrology tool; acquiring a target spectrum for the sample using the optical metrology tool; generating a differential experimental spectrum based on a difference between the target spectrum and the anchor spectrum; and determining the parameters for the sample based on a comparison of the differential experimental spectrum to a differential simulated spectrum, wherein the differential simulated spectrum is based on a difference between a spectrum simulated using a model having the parameters and a spectrum simulated using a model having anchor parameters, wherein the anchor parameters are obtained from one or more reference samples and are determined using one or more reference optical metrology tools.