1. A method of determining a property of a substrate, comprising: independently measuring scatterometry spectra from corresponding targets on the substrate; linking a variable value for a first variable parameter of a first one of the measured scatterometry spectra and for a corresponding second variable parameter of a second one of the measured scatterometry spectra to reduce a number of variable parameters for a reconstruction of each of the measured scatterometry spectra; and deriving a value for the property based on the reconstruction of each of the measured scatterometry spectra.