We're currently working on a high-rise commercial building, and hope someone could shed some lights on the following questions:

1. Is it ok to calculate the % of rainfall captured and treated based on areas? Say, the total site area represents 100% rainfall, and there's 50% of building footprint collecting rainwater and treating all that's collected, with another 40% of landscape able to capture all rainwater within the area. Can we assume that 90% of the rainwater within the site is captured and treated?

2. In this case, the high-rise building will not only be able to collect rainwater from the horizontal surface on the rooftop, but also the vertical surface which blocks some of the rainfalls and leads them to a mid-level gutter system. Does the vertical surface also contribute to the 90% rainfall calculation?