This contribution proposes a design methodology to realize Fabry-Perot cavity antennas with enhanced aperture efficiency and bandwidth. The proposed antenna employs a tapered Partially Reflective Surface (PRS) and leverages a com-bined approach incorporating principles from the simplified ray model and two-dimensional leaky-wave theory. By employing a transmission line model of the PRS, unit cells are selected that simultaneously satisfy the resonance condition, present a positive reflection phase slope, and synthesize the necessary leakage factor function. A design to obtain a uniform aperture distribution is carried out. The simulation results are compared against the best design from a previous study, thus showing the improvement realized by incorporating leaky-wave theory into the antenna design process.