90 % of the average annual rainfall

Could anyone give me an advice which of the following interpretation complies with the credit language? There are two possibilities how to construe this requirement: a) 90% of the average annual rainfall is equivalent to 1/0.75/0,5 inches based on climate as states in LEED Reference Guide. As explains Gregory Hurst down here (Aug 26 2011), this is the depth of rain that equates to 90% of the daily rainfall amounts, 90% of the days will have rainfall amounts under the calculated value. From this point of view 90% determines only the rainfall depth. For example if I have 1000 sq ft of absolutely imperviousness site in humid climate, I must capture and treat 900 cu ft of rainfall (1000 sq ft * 1 in) to meet the credit intent. b) The numbers of rainfall depth 1/0.75/0,5 inches represent the average annual rainfall and 90% of the calculated runoff must be treated and captured. This interpretation conforms to the possibility to calculate 90% rainfall compliance based on areas (see Simon S., Nov 29 2011). In this case if I have 1000 sq ft of absolutely imperviousness site in humid climate, I must capture and treat only 810 cu ft of rainfall (0.9 * 1000 sq ft * 1 in) to meet the credit intent.