Don, I don't know what chart you are using, but the remaining capacity based on the resting voltage can vary considerably between cell manufacturers
(Panasonic, Samsung, Sony and LG etc.) and can also vary significantly between different cell types from the same manufacturer. LG alone have produced more than
60 different types of 18650 cells.
It is impossible to accurately determine the remaining capacity based on the cell voltage without knowing the exact cells used in your battery, and having a specific chart for that particular cell showing the capacity at different resting voltages. The temperature of the cells can also affect the remaining capacity at any given voltage.
The actual low voltage cut off will be dependant upon the battery's
BMS. I don't know what the preset minimum cell voltage for your
BMS is, but if it's programmed to cut off when any cell drops to 3.0V per cell, then your battery would cut out at 42V
if all your cells are perfectly balanced. However, if your cells are not well balanced, or you have a weak cell group, the
BMS will cut the power when the lowest/weakest cell group drops below the preset cut off voltage, but your battery voltage would still be higher than 42V due to the remaining stronger cells being at a higher voltage.
As an example, if you had a single weak cell group that dropped to 3.0V while all the other cell groups were still at 3.7V, your battery voltage immediately before cutting out would still have been 51.1V.
As this 51.1V would have been measured while the battery was under load, the actual battery voltage would be even higher as the cell voltages will typically rise after the load had been disconnected by the
BMS.
Alan