A thermodynamic band gap-based model for mixed-halide perovskite photosegregation currently explains numerous features of the phenomenon. This includes excitation intensity (Iexc) thresholds for photosegregation as well as Iexc-dependent photosegregation rates and rate constants. An intriguing prediction of the model involves Iexc-dependent terminal halide stoichiometries (xterminal), following photosegregation. Rather than suggest a common terminal value, e.g., xterminal = 0.2, for methylammonium lead iodide/bromide [MAPb(I1-xBrx)3], the model predicts Iexc-dependent xterminal. This, in principle, allows for the controlled tuning of mixed-halide perovskite photoresponses. More important, though, is the opportunity to study this response to develop deeper insight into the origin of nearly ubiquitous photosegregation in lead-based, mixed-halide perovskites. Here, we demonstrate Iexc-dependent xterminal in formamidinium/cesium, mixed-halide [FACsPb(I1-xBrx)3] perovskites. We show that modifications to theory, which account for photosegregated domain subpopulations and photocarrier funneling efficiencies, lead to good agreement between measured and predicted Iexc-dependent xterminal values. I-rich phase fractions increase with Iexc and result in asymptotic xterminal versus Iexc. This addresses an open discrepancy between experiment and theory to advance a detailed understanding of light-induced instabilities in mixed-halide perovskites.