Are you also checking your SG and logging it?
Battery monitors are very common even with lead-acid batteries. They involve a shunt installed at the battery (-) through which all charge and discharge current passes. The device counts the net current and direction and provides an estimate of the SoC based on the input total bank capacity.
They're spendy, but I violently recommend the Victron BMV-7XX.
700 is the base model
702 allows for battery temperature monitoring OR for a second battery voltage.
712 is the 702 but with bluetooth and VictronConnect access (highly recommended)
You enter all of your battery bank parameters and charge termination criteria, and it maintains a running count. When it detects that the full charge criteria are hit, it synchronizes to 100%.