A minor typo error shows the pipe diameter, 2.5", being squared times pi for a result of 4.9 sq in. I am sure you intended pi(1.25 x 1.25) = 4.9 sq in. the cross sectional area of the 2.5" dia pipe.
More importantly for me is the clarification and comparison you so clearly establish to allow accurate comparison of data from different sources. Thank you .
Now I must return to my calculations and use the knowledge you just gave me to make a better choice for my 496ci Caddy BB exhaust system.
http://www.dynomax.com/ecat/pdfs/ultraflowelded.pdf
http://www.magnaflow.com/02product/universalsatinxl.asp