After a good night's sleep the chart you reference makes more sense. It is used when load testing the battery.
The most common load test is the 20 hour load test. To do this, divide the ah capacity by 20 and apply that load. For a 100 ah battery the load would be 5 amps, thus after 20 hours at 5 amps 100 ah would be used and the battery would be at 100% DOD or 0% SOC. The chart shows the estimated voltage readings along the way.
Using the 20 hour rate again, when the voltage reading is 12.65v the battery is at 90% SOC or 10% DOD, when it is at 12.15 volts it is at 50%, at 11.83v the SOC is 30% and DOD is at 70%. Finally when the voltage reaches 10.5v it is completely discharged.
The other columns are for different loads. The 8 hour rate for 100ah battery would require a load of 12.5 amps applied for 8 hours. The voltages listed are for the intermediary stages, 10% DOD, 20% DOD, etc.
The manual suggestion of a 25 amp load is based on a 10 hour test, the results of which are not shown on the chart.
The second chart on page 32 of the manual, gives the information that
@Davidasailor26 was referring to. On that chart if the OCV is 12.3 the battery is at 60% SOC. This is a resting voltage with no loads or charging for at least 4 hours. David is correct in advising not to go below a 50% SOC charge and to return to 100% as soon as possible to prolong battery life.
A key idea is the difference between resting voltage (Open Current Voltage) and voltage under load. All battery voltages will drop when under load, the greater the load, the greater the drop. Once the load is removed the voltage will rise and it is that voltage (resting) that indicates the SOC. In your scenario, drawing 50 amps for 1 hour from a fresh fully charged 250 ah battery, 20% of the capacity will have been removed, according to Chart 2, the battery should read about 12.54v.
The first chart, will give you an idea of how long the 50 amps can be drawn from the battery. (although it would require extrapolating between the 8 and 20 hour rate). Making the math easier, a 250 ah battery with a 30 amp load will last about 8 hours before it reaches 10.5 volts. Decrease the load to 12.5 amps (250/20) and the battery will go 20 hours before it reaches 10.5 v.