12.7 = full charge
12.3 = half
12.0 =near enough to nothing left
These are only approximate and the industry uses a 10.5V figure to indicate the battery is deemed to be no longer doing what it should.
The actual numbers do depend also on temp, see Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 4 for some harder data.
Charging voltage ideal, 13.8-14.4, some newer types of batteries demand higher charging voltages.