That's an interesting graph of state of charge, SOC, vs discharge rate, and appears to be at a constant load (with various load rates graphed). How does that relate to varying loads?
SOC of a lead acid by voltage reading is normally determined at rest, and for an accurate SOC at rest for 12-24 hours. When the load goes to zero, or some small %, then the battery will rise to a voltage less than true reading of SOC. This can be a good representation of SOC with a somewhat small bias short of real SOC as long as you take the reading in the same way consistently.
Before I had an Amp-Hour meter I just had two readouts, voltage and current. By reading the voltage a time after the current dropped to the minimum value (assuming there were always some small loads) I would get a good idea of the SOC. It seemed to be within a few percent, but the main thing was to see that I did not discharge too deeply.