except that manufacturers account for that and calibrate such that what you think are 0 and 100% are actually closer to 20 and 80% respectively.
It’s actually the other way around. A safe cutoff voltage to prevent battery degradation is about 4.2V, but most modern phones charge until 4.45V, so they can advertise a bigger battery capacity at the cost of long term battery health. Your phone essentially charges itself to more than 100%
There are sadly no worthy FOSS alternatives, at least for real work. But for creating memes gimp would be plenty