A lithium ion battery is optimally stored at 3.8V per cell. For the MacBook this means 11.4V for the battery.
When discharging from a full charge, 11.4V are around 40%. After one month with battery limit at 40%, the battery voltage has dropped significantly. 11.4V are around 60% now.

Currently, I adjust the percentage limit every few weeks to keep it at the desired voltage of 11.4V.
It would be very nice to integrate this limit by voltage into a tool, so that the battery state of charge stays correct, even when the percentage indicator begins to drift.

The voltage can be retrieved using:

ioreg -l -n AppleSmartBattery -r | grep "\"Voltage\" =" | awk '{ print $3/1000 }'