FABRE (Fan Augmented Air-Breathing Ramjet Engine) represents a hypersonic combined cycle power system with extensive applications in the aerospace industry, particularly in TSTO scenarios, capable of achieving speeds up to Mach 7. However, a persistent challenge lies in achieving optimal flow matching at high Mach numbers within the FABRE dual-mode scramjet combustor, characterized by a low length-to-diameter ratio. To address this challenge, this study employs numerical simulations to scrutinize the flow dynamics within the FABRE ramjet combustor across varying geometric throat areas at Mach 7, culminating in the derivation of a matching law between the geometric throat and the isolator. The findings underscore that insufficient throat area causes inlet issues despite efficient combustion. At 89% throat area, combustion efficiency peaks with improved total temperature. Optimal efficiency is around 89% throat area. Throat area from 22% to 67% enables subsonic combustion; beyond 67%, it shifts towards supersonic combustion. Subsonic combustion sees reduced thrust and specific impulse with increased throat area, while supersonic combustion shows the opposite trend. Prioritize the 22% throat area for performance or the 100% for lower thermal protection needs. This investigation elucidates crucial insights into the geometric throat matching characteristics of hypersonic combustors, thus furnishing valuable support for future advancements in aerospace propulsion systems.