Calculating AH usage Based on SOC

Trending state of charge by voltage is difficult. First a thorough baseline needs to be established in which a discharge amp hours curve is plotted. I've seen fuzzy logic voltmeter "state of charge" meters come and go but the bottom line always turns out to be "Best Guess". Folks who regularly cycle their batteries using fuzzy logic regularly purchase replacements more often than other folks who have a better grip on the reality of where their batteries are at. Try as I might, I have never achieved the ability of staring at a beaker of electrolyte and prestidigitating it's specific gravity. Voltage is a derivative...

"There is no cop behind me with his red lights on, so therefore I cannot be speeding"