Fundamental derivation of MAPbX3 optical constants is paramount toward the development of high-performance electronic and optoelectronic perovskite devices. In this report, we present a rigorous optical dispersion data analysis of single crystal MAPbBr3 via variable angle spectroscopic ellipsometry appended with transmission intensity data. This results in a robust derivation of MAPbBr3 optical constants for normal and anomalous optical dispersion regimes. Using the derived optical constants for our single crystal MAPbBr3, exemplary modeled solar cell optical device designs are optimized for perovskite layer absorptance. Our optimized PSC designs demonstrate improved optical performance in comparison to designs prepared using illustrative MAPbBr3 literature optical constants reported to date.