Magnetic activity similar to that of the Sun is observed on a variety of cool stars. Strong magnetic fields in stellar interiors and produce many magnetic phenomena, including starspots in the photosphere. These starspots are a major source limiting the precision of high-precision radial velocity measurements used to characterize exoplanets. In a previous series of papers, we demonstrated an empirical approach to measure starspot properties using observed spectra of active and inactive stars as proxies for the spotted and non-spotted photosphere. These empirical methods were successful for heavily spotted stars, but they are not sensitive enough to unravel the jitter caused by numerous small spots. To overcome these limitations, we have evaluated a wide range of model atmospheres and spectrum synthesis codes, and we have assimilated suitable line lists. By characterizing composite (spot+nonspot) synthetic spectra from the entire range of parameter space for G and K stars, we have developed new diagnostic indicators that can be used to measure starspot properties from observed high-resolution spectra of spotted stars, including spot temperatures, areas, and the effective decrease in gravity caused by strong magnetic fields within the spots.