I am working on Core & Shell Office Building Bangladesh where my Site Area is 4434.71 square feet.

Building Terrace Hardscape area is 5134 square feet and softscape is 2226 square feet

Ground Hardscape (excluding Building Footprint) is 5248 square feet and softscape is 3153 square feet

I just wanted to calculate Volume of Captured Runoff Pre-Development and Post Development. Prior to development it was a vegetated area.

Please confirm if I am using the formula correctly: 

Pre-Development Phase:

Impervious of Collection Surface (I) = 100

Average Rainfall Events (P), inches = 1 (as it lies in Humid Climate as per the Table 1.1 given in manual)

Volumetric Runoff Coefficient (Rv) = 0.95

Area of Collection Surface (A), square feet = 14434 square meter

Volume of Captured Runoff (Vr), cubic feet = (P x Rv x A) / 12 = 1142 cubic feet

Post Development Phase, I just wanted to ask whether I need to consider only the hardscape area of roof to calculate or I need to calculate using the four types of area as mentioned above and do a consistent calculation as above and sum up the four obtained values of Vr in the end.

Also elaborate Average Rainfall Events (P) by definition and let me know if its value is taken from some other source like meteorological department or from the given table in LEED Guidelines v2009 on Page 104.

Thanks