IMO it's not an exact science.
My understanding is that the normal LiIon 2-series-2-parallel 4000mAh packs are not harmed down to about 3 volts per cell (6v for the pack). I think SD was being careful in choosing the 6.3v shutdown level for that reason. Also, I don't think the voltage vs. run time is very linear, and the quality of cells in the packs differ. Most seem to start going bad after a few years anyway. Genuine Sonys seemed to last longer, but they don't seem to make them anymore. I try to run on AC whenever possible (which is the majority of the time), but about once a year, I do a "run until it stops" battery test and make a note of it.
You can get humongus packs that fit the Sony L format.