If you really want to get into the subject I have relied on the following article:
https://pssurvival.com/PS/Batteries/...ltage_1993.pdf
Notice the "C/xx" at the right side of each curve. This is to match the particular curve to the rate of discharge / charge in Amps for a given
battery capacity.
This is explained in detail at the bottom of the first page (#67).
I suspect that this model is the basis for any non- AH measuring
battery monitoring system. One can see, with a bit of study of one's own system, it is possible to accurately judge the SOC of lead acid
batteries with a good voltmeter and a bit of knowledge about the Amperage loads of the boat's
equipment. It works for us.