The way that we have calculated run-off volume is to calculate a weighted average run-off coefficient, as you would for the Rational Method. You do this for pre and post development conditions, and essentially, the Run-off Coefficient needs to be the same. For instance, if the Run-off Coefficient is 0.35 for an undeveloped site, and the Run-off Coefficent for the developed site is 0.70, then the volume that must be mitigated is the difference in Run-off Coefficient times the rainfall for the one-and two-year 24 hour design storms. The approach to meeting the volume requirement can be with green-roofs, porous pavment, cisterns and reuse, infiltration basins, bioswales, etc.