Rainwater collection and reuse on campus

Hello, Our project is a new building on a school campus. The Engineers are proposing collecting rainwater from the building's roof, storing this is a big common water tank next to the building, and then reusing this water for flushing purposes for all buildings on campus. So let's say the project building is collecting 200,000 gallons per year, and another building is collecting 300,000 gallons per year. These 500,000 gallons will be stored together and pumped to 3-4 buildings on campus (including those which they were collected from). How can the collected rainwater contribute to the LEED water calculations of this project? The Engineers are confident that all the collected rainwater will be used on campus, but they cannot be 100% sure all the collected rainwater will be used specifically on this building. Any tips on the best way to proceed? Thanks!