The peak voltage of the battery might be 4.2V, but the average will be around 3.6 nevertheless. The voltage before the energy enters the battery doesn't change

The peak voltage of the battery might be 4.2V, but the average will be around 3.6 nevertheless. The voltage before the energy enters the battery doesn't change the calculation of battery capacity (and its time to discharge) anyway. EDIT: now I noticed that the calculation is about charging, rather than discharging. 4.2V makes better sense in that case.