Here is a little chart that I was sent by Batterystuff.com which shows the relationship between measured voltage and the remaining level of charge of a typical lead-acid battery. I will also add this chart to the Hall of Wisdom section:

State of Charge Voltage
100% 12.7 - 13.2
75% 12.4
50% 12.1
25% 11.7
Discharged 0 - 11.6