I bought an extended battery for my ibook a while back, they always quote hte % figure in comparison to the worst battery apple ever shipped though.
For example my g3 ibook's original battery was something like 4200mAh whereas the ones apple was shipping at the time for g4 ibooks were like 6000 odd and the extended ones OWC were selling were only a bit better than new apple ones so i just bought a replacement from the local apple store and it lasted like 7 hours.