A method of controlling a polishing operation includes obtaining a sequence over time of measured spectra with an in-situ optical monitoring system during polishing. For each measured spectrum from the sequence an optical model is fit. The optical model includes dimensions of a repeating structure and the fitting includes calculating a output spectrum using diffraction effects of the repeating structure, and parameters of the optical model include an endpoint parameter and a parameter of the repeating structure. The fitting generates the sequence of fitted endpoint parameter values, and at least one of a polishing endpoint or an adjustment of a pressure to the substrate is determined from the sequence of fitted endpoint parameter values.