agree that manufacturers seem to have different ideas about soc. i have flooded batteries. both trojan and interstate have soc charts
and they're both different.
i've been using voltage as a way of checking soc for many years now and i think i've gotten as much life out of my batteries as anyone else. sometimes i measure specific gravity, but you can't do that with agm's. you don't need to know the EXACT state of charge, just keep it in a high range. i doubt that most cruiser ever get near 100% soc.
here's the trojan guide, which includes an soc table...