We have a tall office building that has a cistern for rainwater harvesting. I have two related questions:
1. The amount of runoff reduced by stormwater harvesting system is based, I understand, on its storage volume, the rate at which the system is emptied, and the interval between storm events. How can I calculate the interval between the storm events? In the reference guide, example 2 "volume of captured runoff" the design storm interval is 3 days. Can the team use this as the default value for the project?

2. The volume of captured runoff is based on the area of collection surface. Considering that the building has terraces, does the team count the horizontal surfaces of the terraces in this calculation?