16% isn't enough. You need to discharge closer to 75% of the battery because the state of charge curve is not linear. I tracked discharge from 100% down to 15.7% (using ~84% of the battery), and my capacity calculation settled around 64 kWh starting at about 24% SOC (using ~76% of the battery).I will do some napkin math like the WIND-WORKS article to estimate my battery capacity (I've seen a youtuber do this too for calculating battery degradation). In the future, I will try OBD Fusion (no Android for me).

I am guessing that one needs to discharge 16 percentage points or more of battery to get a relatively accurate battery capacity measurement with the napkin math. This value can be found by analyzing the overall behavior of the round off error in the displayed SOC on MyChevrolet (the same can be done for kwh spent value, but if you spend 16%+ of your battery, this value is of less concern!).

If I had only used ~16% of the battery, my capacity calculation could have been 62.4, 65.8, or 63.8 depending on which particular data point I used.

Part of the reason is that the SOC displayed value appears to be "sticky" - it will change in discrete 0.4 kWh increments. Obviously, that can lead to dramatic shifts in your capacity calculation if 16% = 10.2 kWh, 9.8 kWh (10.2-0.4), or 10.6 kWh (10.2+0.4).