I wouldn't consider the method posted above to be the absolute method. There have been various threads over the years that the rate constant used in the energy graph does not necessarily match the rate constant used for the rated miles and the calculations in the trips tab.
In reality, my experience is the method quoted above is no more precise or accurate than simply taking the displayed rated miles at a fixed SOC and then ratio to 100%. If you want to be picky, the key is to watch out for precision on the SOC value displayed as it's only displayed to 1% increment. That's where doing either calculation at a higher SOC will be more consistent than if you do it at a lower SOC. The most consistent way is to charge to a fixed percent, say 80%, and then use the rated range value shown within a very short time after charging stops.
In the end, what you're really interested in is what's the trend more than any absolute numbers. Provided you're consistent in the method, realize no approach is 100% precise, you'll be able to understand trending. But even that is subject to estimation errors/how well the BMS system is calibrated, and how well the battery pack in balanced.
Most important is to realize that all of these are estimates. There is no absolute way to measure battery capacity short of very controlled conditions in a laboratory. The BMS system estimate is in my opinion quite good, but it's not an absolute.
Just my two cents from someone who has been tracking data on his MD90D for over 5.5 years now.