The reason voltage alone is not a great indicator of state of charge is the voltage at any given state of charge changes with current. Delivered amp hours will also vary based on current. So state of charge is relative based on those operating conditions, as well as age and temperature.
A very simple rough estimate of state of charge can be determined by measuring the voltage when operating a load (delivering current) and using the following scale:
12.5V - 100%
12.0V - 75%
11.5V - 50%
11.0V - 25%
10.5V - 0%
The 50% rule is a myth. The impact to service life changes little between 40% DOD and 80% DOD and when compounded with all the other factors that affect service life, impact from DOD is lost in the noise. You paid for those amp hours, so use them.
No matter how much or little you use a lead acid battery you want to restore the charge as soon as practical. Don't worry too much about using it as much as you want, that's what it's for but between uses you should keep it fully charged. A battery tender/maintainer takes care of that very well.
Mark K5LXP
Albuquerque, NM