If you typically use 20% and throw away 80%, then using the 80% would extend your battery life by 4x over the original. So 400%.
I'm very skeptical of this.
(4x PLUS the original 1x for 5x total).
I agree with you, just arguing a technicality.
I'd tend to read "extends battery life 800%" as 100%+800%, but they left out the word that would specify one or the other.
Still, I'd interpret the numbers the same way as I would "extends battery life 50%," which I can easily imagine someone writing.
If you typically use 20% and throw away 80%, then using the 80% would extend your battery life by 4x over the original. So 400%.
I'm very skeptical of this.