Better than an exfiltrator trench would be to use a Bioretention area (Rain Garden) This would not only infiltrate the runoff, but provide some treatment as well. If you go to Yahoo or Google and search for bioretention you will get numerous sites describing this method. The bioretention area...
A drainage area of 2 acres of paved parking lot will generate about 1.2 cfs for 0.50" 24 hour rainfall. A 6" orifice with a 2' head will discharge 1.25 cfs. CHeck the inflow. If it is less than 1.25 cfs, it will all go out the 6" pipe. Set the invert differential between the...