I think you are asking a bit much of GIS. Calculating the runoff volume is more of a hydrology function. Arc GIS and Arc Hydro don't perform hydrology calculations. They can help gather the data needed for hydrology (area, watershed slope, longest flow path, etc.). You've already done that it seems.
It may be that the data you've collect is enough to calculate the runoff using a simplistic method such as the Rational Method. Your local jurisdiction (County or City) should be able to direct you on what procedures you should use. They would likely have runoff coefficients for you
If you want a hydrograph, where the watershed is located could determine what standard you are using for the storm depth, duration, storm rainfall pattern, and a few more things. There are several methods for determining the rainfall/runoff. The local jurisdiction (County, City, State or Federal agency most involved in that watershed) may have a standard you need to follow to determine the flow rate at the pour point.
So, I would find someone in your watershed's area who knows the what the hydrologic standards and methods are and follow their direction. A free program named HEC-HMS (HMS) created by the Corps of Engineers could likely be used for whatever standard they specify. ESRI has developed HEC-GeoHMS for the Corps to help transfer the input parameters from GIS to HMS. The GeoHMS process is somewhat complicated if you don't know GIS and HMS well.
Hope this informs/helps.