Lead battery capacity

In the southern french alps, my standalone system is fully managed by Victron: 2kWp panels by blue solar MPPT, lead batteries by BMV712, Generator by Multiplus, load mostly 230V.
My lead batteries are 12 OpzS 2V and total to 30kWh (1260Ah C10, 1714 Ah C120, 1373 C20). They are 12 years old and power draw never above 33%.

I tried twice to measure the remaining capaciy by shutting the incoming power from the panels, drawing as much as possible from the multiplus for a limited time then linearly extending kWh ant time to 100% SOC. Résults:
in 27 hours Soc from 92 to 67%, gives total capacity : 40 kWh C108. In 14 hours soc from 92 to 86%, capacity 52kWh C 233.

I am not sure of using peukert formula as the index may change overt time.

Is my method correct?
Any other method?
Am I correct to infer my capacity is well under the initial 30kWh?

Thanks for your light (12, 24 ou 230V)

OPZ ?
Have a look in the forum:

For capacity check you can not calculate from SOC or kWh, you need to quote the Ah consumed in the test period. The shunt should provide this information. The SOC calc includes Peukert factor calcs and kWh varies with voltage so neither are useful for this evaluation.

Thanks!
Will have a go, But Ah does not give me the actual capacity!
Thatquestion may be closed!