Clay,
 
Attached is a copy of the revised Drainage Area Map with new calculations and a spreadsheet for the Detention and Orifice calculations. I used the one furnished from Chris Robinson and
revised it to reflect the added area between the edge of the taxiway and the lease line, which is now being paved. 
 
Basically, the existing discharge from the unimproved area of 3.01 acres is 6.8 cfs. The undetained part is 3.1 cfs thereby reducing the allowed release to 3.7 cfs. The allowed pass
through the detention is 4.3 cfs resulting in a total allowed release of 8.1 cfs (after round off).  
 
I also lowered the starting hydraulic grade at the connection to the existing inlet. It is now the inside top of the new 24" pipe. 
 
This produced a detention volume of 24,046 cubic feet and an orifice opening size of 13.7 inches. 
 
Please have Chris review this information to see if he agrees and contact me with any questions or comments.  
 
Regards,
L. Lynn Kadleck, PE, RPLS
Kadleck & Associates
555 Republic Drive, Suite 115
Plano, Texas 75074
972-881-0771
lkadleck@verizon.net