Well, this is obviously not an exact science.
I think the basic technique still makes sense from the standpoint of roughly assessing where you may be on the curve towards battery 'failure' (good, fair, poor, or similar). If you want to know your battery capacity with greater precision than that, then I suggest that you need to be watching the trend from a large number of charges. Furthermore, it is better if the data is from the same
EVSE setup (eliminates further variability from a multitude of charger setups and operating currents). I felt like the graph I showed earlier (a composite of 200 charges) allowed me to do this average quite well just with the eyeball. All of those points were with the same EVSE setup so the 18 month trend is very apparent and believable.
At least this simple method can give you a feel for whether it is worth a trip to the dealer for a readout of the actual capacity. I assume the vehicle capacity report also includes some kind of filtering such that you would not get a different number every time you pulled it.
Ultimately everyone here would dearly love to be able to pull the number from the vehicle ourselves. There is currently a $380 tool that can do this, and there is hope for a $60 tool with future software updates.
@Cash Traylor is working on coming up with something on his own using a Panda. If we eventually get an affordable solution to read out the actual capacity, this all becomes moot.