The range of a near-infrared (NIR) calibration model for tablet assay is extended using production “seed” spectra and synthetic spectra generated from placebos and “pure” API spectra. This method resolves the problem of not being able to obtain a range of production samples with +/- 15% of the label claim of API concentration online. A synthetic model is developed using five real production samples and 70 samples synthesized from fractions of “pure” API spectra added to placebo spectra. A synthetic calibration model with an R2 = 0.9987 and a standard error of calibration of 0.0037 mg was developed with this method for an API with 1.0 mg label claim in a 100 mg tablet.