If the top of your charge level is the "last 20%", then you can have two or three or four or more "last 20%"s by buying a bigger battery.

I'd say the opposite, that you're losing the first 20% as your battery degrades, which is the least important part. And if you need a specific range for some trip, again just size your battery appropriately. Make sure 80% or 70% of the brand new range is enough for that trip.