Our project is an adaptive reuse on an urban site that was previously about 95% impervious. We are increasing the area of vegetated space (the old parking lot is being demolished), collecting rainwater from about 6700 SF of the roof area, and creating a rain garden out of a vegetated parking island that is about 1800 SF. We had already put together the stormwater volume reduction calculations using the NRCS method before the team decided to add the rain garden (we had previously been considering pervious pavement for a section of the front driveway), and now we are having trouble determining the proper way to quantify the runoff that will now be directed to the rain garden in the calculations. We plan to slope a section of the paving which surrounds the rain garden towards the rain garden, but how should this be quantified? Should the volume of water which will be draining to the rain garden be subtracted from the Runoff Volume Total in the design case (assuming that we have properly accounted for the rain garden being able to handle that 2-year 24-hour design storm volume based on the inches/hour infiltration rate for the soil), or should the CN value for that section of the paving be changed?