I had never bothered to check the battery voltage other than to check that the charger and BMS were working properly and bringing it to a full charge. When my old 36v pack would no longer maintain 42v I replaced the pack with a unit of higher capacity.

Yesterday I happened to return from a ride with the display indicating the battery level at about 50%. This thread had made me curious so I checked the voltage immediately before there was any significant rebound. The pack measured just over 36v which is pretty much in line with the 50% level on the chart posted by the OP. I'll give it a few months and check again for comparison.