LEED CI in a LEED CS building - WE

We are going for LEED CI v2009 in a LEED CS v2009 building. Both are in design. Our client (LEED CI) will be in more than 75% of the building. There are no restrooms in our tenant fit-out (restrooms are in the base building) but there are several pantries throughout the tenant space. If we encourage the base building to incorporate a rainwater harvesting system into the water use infrastructure to achieve a 40% water reduction, does this mean that we can piggy-back on their reduction? I've noticed that both WE forms for CI and CS do not incorporate rainwater capture systems into the water reduction calculation. For CS/ NC, under 'Implementation' in WEp1, rainwater collection is mentioned as a strategy to consider. How do project teams incorporate rainwater harvesting into the water-use reduction calculations? Can we use this water reduction calculation in our LEED CI pursuit? Many thanks.