I have forgotten the precise numbers right now, but you can dial in your amp hours and corresponding percentage of state of charge by noting the voltage. IIRC, a 50% state of discharge occurs at a bit over 12 volts...like 12.2. So not only can you determine a relatively true amp hour capacity of a new battery, but you can adjust for aging ones as well.
I need to look up the precise numbers, but that's pretty close I think.
"If the women don't find you handsome, they should at least find you handy." - Red Green