Cistern in only one building in campus master site - points only for that building?

I'm working on a complex mixed-use Campus project, where we'd like to pursue this credit for 1 of the separate buildings.

Only one of the buildings on the master site will have a cistern that we will use to manage rainwater on site for 2 points. I am not clear how to approach this credit if only one building has a cistern for rainwater management, within the Campus context:

- Should I use the whole master site area and all roof areas to calculate runoff volume, even if only looking at 1 building?

- Can I pursue the credit for only 1 of the buildings?