Phase Change Materials (PCMs) enable thermal management of high heat flux systems through their high latent heat of phase change that damp temperature rise in the system. Although numerical simulations can be used to design such systems, for an efficient thermal design process, it is desirable to develop a Figure of Merit (FoM) for quick quantitative comparison of performance without detailed simulations. The previously developed FoM for PCMs only depends on the thermophysical properties of the PCM and does not account for geometry. But geometry impacts the selection of PCMs for optimum performance due to the balance of thermal conduction and thermal energy storage. In the present work, we develop a novel performance factor (f) to modify the FoM based on the characteristic timescales for thermal transport and thermal storage to gauge the thermal management performance. The applicability of this metric was validated with transient numerical simulations with the metric of the maximum temperature (Tmax) in the domain after a fixed heating time. Across various geometries, the trend of decreasing Tmax with increasing f is clear for both selected organic and metallic PCM (PureTemp53 and Alloy136, respectively). Moreover, as opposed to the the single conventional FoM for a given PCM independent of the geometric details, the performance factor f can be used to identify ideal geometric configuration based on their thermal response for the same PCM. The proposed approach can predict which combinations of PCM and geometries are ideal for managing temperature rises in systems without detailed numerical simulations.