In April of 2010 I filled my 2009 ULA Ohm with packing peanuts, mushed it around until it looked typical (versus optimally barrel shaped), poured the peanuts into a tall slender box that approximated the shape of the Ohm, measured the depth, multiplied by the area and voilà - it held 2550 ci total, of which 600 were in the extension collar. That doesn't include pockets. That thread can be found Here.
When BPL did their June 2011 Lightweight Pack article they found the ULA CDT was over rated by about 25%, using the same method. The details can be found in this Archive.
Now you need someone to do that with the Porter.
Otherwise it's all smoke and mirrors.