The sport of baseball has a strong tradition of utilizing forward-projecting probabilistic measures, such as batting average, for performance assessment. Albeit highly informative and quite accurate, such measure of batting probability merely looks at how a player can probabilistically act upon the system in a forward manner, based on how they have acted upon that system in the past, but does not provide information about the state of the system itself once a player completes an at-bat. The current paper proposes a statistic called the Residual Run Potential Index (RRPI) as a measure of the ‘residual system state’ as it remains after a completed at bat. Derived from Retrosheet (2017) event data, the RRPI reflects a system-efficiency measure of player hitting style that retains maximized runscoring potential of the system state. RRPI calculations derived from a proprietary software simulation experiment, as well as for several Major League Baseball (MLB) Hall-of-Fame players, are also presented.