1. A method of measuring parameters of a sample with an optical metrology tool, the method comprising: illuminating one or more reference samples with light from the optical metrology tool; acquiring one or more spectrum from the light after interacting with the one or more reference samples using the optical metrology tool; determining an anchor spectrum for the optical metrology tool using the one or more spectrum acquired from the one or more reference samples using the optical metrology tool, wherein the anchor spectrum is a reference spectrum that is used to generate differential experimental spectra for all samples measured with the optical metrology tool; illuminating the sample with the light from the optical metrology tool; acquiring a target spectrum from the light after interacting with the sample using the optical metrology tool; generating a differential experimental spectrum based on a difference between the target spectrum and the anchor spectrum; fitting 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 variable parameters and a spectrum simulated using a model having anchor parameters, wherein the anchor parameters are determined for the one or more reference samples using one or more reference optical metrology tools; and determining the parameters for the sample based on the variable parameters in the differential simulated spectrum that is a best fit to the differential experimental spectrum.