SoC of lead acid batteries based on voltage is merely a guide for NEW condition batteries. It is also different for wet cell , and AGM and gel cell battery chemistry.
It can't even be considered a reliable reference if the battery has had some use and shed active plate material or developed any sulphation.
A lot of OTC chargers intepret the SoC from the battery Voltage and often undercharge batteries resulting in creeping conversion of active plate material into sulphated plates and loss of capacity.
I have processed almost 200 defunct lead acid auto batteries and data logged their behaviour including building a custom data logging Ah capacity load tester now on sale at Clubjameco as a DIY kit.
Here it is open sourced.
https://hackaday.io/project/1375-lead-acid-ah-capacity-testerlogger
Identical battery types (used) can have vastly different discharge curves and knee inflection points. Battery chemistries (Pb-Pb; Pb/Ca;Ca/Ca; Pb-Se/Ca; Pb Sn, Ca etc.) affect their loaded performance due to sulphate nucleation sites 'spawning' or 'enabling' faster discharge sulphation which shows up as a bump in the voltage discharge curves shortly after discharge has begun. Then there is the issue of the % of tetrabasic vs tribasic plate material for batteries that offer both starting & deeper cycle performance. This mix of plate material also charges voltage behavior.
If you wish to develop some sort of reliable SoC monitor it will need to be one that logs charge in/out and every few months you do a relatively deep cycle discharge to characterize your system based on battery aging. SLI batteries can't tolerate more than a 50% deep discharge w/o losing plate material and degrading longevity. I believe products like this exist for the marine industry. With so many battery variants available, you must characterize each battery to profile its SoC before the monitor can be useful.
There is a lot of information on these matters online but it takes significant research to develop a handle on how these batteries actually work given all the improvements made in the last few decades which affect apparent SoC.