The problem with measuring the battery state of charge under load is it will be different depending on the load. The battery has internal resistance, which causes voltage drop dependent on load.
For example, I often draw 60 amps (for a total of 10 amp/hrs) from a pair of 6V batteries while running an inverter making a pot of coffee. During the draw the voltage drops to 11.75, however as soon as the draw is done it jumps back to 12.5 or better.
For no load, or a very light load the table Roy will give a pretty accurate indication of SOC, but as the load increases it gets less accurate.
There are metering systems such as a
Tri-Metric that measure actual battery amps out & in, which will give you a more accurate indication of the battery condition than a simple voltage measurement.
