Voltage vs SOC for Flooded, Gel & AGM Batteries
From the East Penn technical manual for Deka Dominator gel batteries
Open circuit voltage is inferior to specific gravity as an indicator of a lead battery's state of charge (SOC). But you can't measure SG with closed (valve regulated recombinant, or VRR) batteries, so voltage is all you have to go on.

The table below describes SOC estimates for a 12 volt battery after at least 8 hours of standing off charge and unused.

 

State of Charge Voltage
Flooded Gel AGM
100% 12.60 - 12.70 12.85 - 12.95 12.80 - 12.90
75% 12.40 12.65 12.60
50% 12.30 12.35 12.30
25% 12.00 12.00 12.00
0% 11.80 11.80 11.80