It looks like this has been talked to death, but there are a couple things worth mentioning.
First, I like the idea of having a battery
discharge chart, but never have one. For me, I just keep the fact that when my battery
hits 12.0 volts, it's half-way discharged. If I go much below that I head into the danger zone where my battery
can become "sulphated," shortening it's lifespan. (More on sulphation below.)
A second thing to keep in mind is you can't accurately measure a battery's voltage when the battery is in-use, which is most of the time when you're checking your battery voltage. DO NOT worry that your battery voltage is well below 12 volts when the furnace
fan is running!
To more accurately measure our battery voltage I flick the meter on for a moment or two when the furnace
is not running and most of our lights
(except for an LED or two) are turned off . . . usually as we head off to bed. That way the draw on the battery is pretty minimal and the reading much more accurate.
Returning to sulphated batteries, something I've taken to doing is putting my battery on a "BatteryMinder" trickle charger during the off-season. The BatteryMinder runs in a maintenance mode that reverses the sulphation of batteries that occurs naturally over time. It's kept our battery in pretty good shape; some people have restored used, heavily sulphated batteries to useful service using a BatteryMinder, making it a pretty worthwhile $60 investment.