A method for estimating a performance property of a naphtha boiling range composition, comprising: representing an optical spectrum of a first naphtha boiling range composition as a first weighted combination of optical spectra from a library of optical spectra; generating a model of composition for the first naphtha boiling range composition based on the first weighted combination of optical spectra; and estimating a RON, MON, or both based on the model of composition, characterized in that the method further comprises: determining a first partial representation of the optical spectrum of the first naphtha boiling range composition as a second weighted combination of optical spectra from the library of optical spectra, the first partial representation being determined by partial least squares analysis, wherein the first partial representation corresponds to a spectral region associated with a first compositional class, and wherein the first partial representation comprises a first leverage value and a first residual value; determining a second partial representation of the optical spectrum of the first naphtha boiling range composition as a third weighted combination of optical spectra from the library of optical spectra, the second partial representation being determined by partial least squares analysis, wherein the second partial representation corresponds to a different spectral region associated with a second compositional class, and wherein the second partial representation comprises a second leverage value and a second residual value; calculating a combined leverage value based on at least the first leverage value and the second leverage value; calculating a combined residual value based on at least the first residual value and the second residual value; and identifying the first weighted combination of optical spectra as having good fit quality based on the combined leverage value being one or less, the combined residual value being one or less, or a combination thereof. representing an optical spectrum of a first naphtha boiling range composition as a first weighted combination of optical spectra from a library of optical spectra; generating a model of composition for the first naphtha boiling range composition based on the first weighted combination of optical spectra; and estimating a RON, MON, or both based on the model of composition, characterized in that the method further comprises: determining a first partial representation of the optical spectrum of the first naphtha boiling range composition as a second weighted combination of optical spectra from the library of optical spectra, the first partial representation being determined by partial least squares analysis, wherein the first partial representation corresponds to a spectral region associated with a first compositional class, and wherein the first partial representation comprises a first leverage value and a first residual value; determining a second partial representation of the optical spectrum of the first naphtha boiling range composition as a third weighted combination of optical spectra from the library of optical spectra, the second partial representation being determined by partial least squares analysis, wherein the second partial representation corresponds to a different spectral region associated with a second compositional class, and wherein the second partial representation comprises a second leverage value and a second residual value; calculating a combined leverage value based on at least the first leverage value and the second leverage value; calculating a combined residual value based on at least the first residual value and the second residual value; and identifying the first weighted combination of optical spectra as having good fit quality based on the combined leverage value being one or less, the combined residual value being one or less, or a combination thereof. determining a first partial representation of the optical spectrum of the first naphtha boiling range composition as a second weighted combination of optical spectra from the library of optical spectra, the first partial representation being determined by partial least squares analysis, wherein the first partial representation corresponds to a spectral region associated with a first compositional class, and wherein the first partial representation comprises a first leverage value and a first residual value; determining a second partial representation of the optical spectrum of the first naphtha boiling range composition as a third weighted combination of optical spectra from the library of optical spectra, the second partial representation being determined by partial least squares analysis, wherein the second partial representation corresponds to a different spectral region associated with a second compositional class, and wherein the second partial representation comprises a second leverage value and a second residual value; calculating a combined leverage value based on at least the first leverage value and the second leverage value; calculating a combined residual value based on at least the first residual value and the second residual value; and identifying the first weighted combination of optical spectra as having good fit quality based on the combined leverage value being one or less, the combined residual value being one or less, or a combination thereof.