Article 22 of the International Telecommunications Union (ITU)'s Radio Regulations, Equivalent Power Flux Density (EPFD) is defined to ensure protection from harmful interference from non-geostationary (NGSO) systems into geostationary (GSO) satellites or earth stations. EPFD depends on the power flux density amplified or attenuated by the antenna gains of the NGSO and victim GSO systems. This occurs when an NGSO satellite transmits towards their system's ES, and some of the NGSO's transmitted power and causes interference to the GSO ES. EPFD calculations are performed in three directions: down, up, and intersatellite. The paper focuses on the EPFDdown algorithm and its applications in interference regulations and radio astronomy services (RAS), which tend to be more susceptible to unwanted emissions from other systems. In order to protect RAS, a new set of EPFD limits was established. To ensure that RAS is protected, the ITU Radiocommunication Sector restricts harmful interference. EPFDdown is defined as the EPFD from the NGSO satellite into the victim GSO earth station (ES), and accounts for the gain pattern of both NGSO satellite and the GSO victim ES in a particular orbital geometry and frequency band. In terms of RAS, RAS telescope sites on Earth are victim ES. To help prevent Article 22 limit exceedances, the ITU's Radiocommunication Sector (ITU-R) defined Recommendation S.1503 to provide guidance for satellite operators developing software to evaluate EPFD limits. This study focuses on implementing the EPFD-down algorithm in ITU-R S.1503 to analyze EPFDdown. With the EPFD validation process, NGSO systems can ensure that incumbent and planned GSO networks can operate without harmful interference, reducing data loss caused by interference from NGSO satellites. By going through each step of the EPFD validation process to validate Mangata's EPFDdown showing, this paper aims to make the complex EPFDdown algorithm more accessible to companies who are operating satellite systems.