Battery Life calculation
As the footnote there says, "The factor of 0.7 makes allowances for external factors which can affect battery life." Presumably this estimate is meant to include a number of factors that will decrease the ideal estimated runtime, such as inefficiencies in buck/boost regulators, and losses from running at higher currents than the standard discharge current used for the nominal capacity rating (typically C/5 for Li-on and C/20 for lead-acid) - see Peukert's law.

But you can typically compute much more accurate runtime estimates if you know the current rate(s) for your device. Then you can use the appropriate discharge curves in the battery's datasheet, and combine that with the efficiency curves for your buck / boost converter.