Hello again,
If i had to depend that much on a battery i would be sure to charge it before each time i used it (boat power or other fairly important use).
I would never depend that highly on a voltage measurement unless it read a very low voltage or a very high voltage, which would tell me either it was fully discharged or fully charged.
If you read some of the better papers on state of charge estimation you will find that it's very hard to be sure of the state of charge for a battery, even li-ion, except for fully charged or fully discharged.