question

Sorry it is a long one. There is a book on battery charging by Victron.

Has the target / bulk voltage been met? If it has then it should be kept there for some time and drop down to the float voltage set. Then you know it is 100% charged. Lead acid will always draw some voltage on float to maintain it there. On a good set between 2% to 4% on the ah capacity Depending on age and chemistry.

The mppts in my experience on voltage readings are pretty accurate when set up correctly. To check your setup measure the battery terminals and compare and the app and mppt battery terminals.

To be honest though lead acid batteries have a 0.3v to 0.5v range to float and charge within so that difference is usually not a big deal unless you are perdantic about things.

A well set up bmv or shunt based measuring system of is the best way of knowing if the power drawn from the bank has been replaced (with extra for conversion and efficiency loss) is the best way to go if you really want to be sure.

Form voltage alone though it can also be a hit and miss as the battery is supposed to rest for a period before measuring.

0% is whatever you maufacturer has deemed to be flat. There is not alot of info on your particular batteries, but from my experience with lead calcium you need a higher charged voltage than other lead based batteries about 14.7v bulk and 13.8v for float (in cycle use) 2A or less to maintain float. 16v equalisation. And 10.8v is dead flat. You can draw lower but will be damaging them really badly then. You basically dont want to cycle them under the 12v nominal voltage for longer life span.

Do you have a battery monitor of any kind connected to the battery bank or are you relying on the charge controller for the state of charge? Voltage is not a good indication of state of charge.