Equation 3 Help

I see that equation 3 in the reference manual performs the calculation for Q, the peak runoff rate, but I do not see how this value is used to calculate the amount of water retained on site. How do you take the storm intensity calculated based on NOAA data and calculate the design storm rainfall volume? Once rainfall volume has been calculated, how do you apply the runoff rate in order to calculate the amount of water mitigated on site? My project does not currently have any special detention facilities in place, and I would like to assess our liklihood of acheiving this point. It seems to me that the reference manual provides very little support for calculating the volume of runoff using equation 3, is this a valid option?