How is the battery degradation report calculated? How do you determine degradation based on a charge to 80% for example?

Here's how I determine the maximum range. Click the Calendar in the menu bar. Look for days when it was charged and click the date in the upper-right of the date cell to open that day's data. In the Charge (green battery symbol) row, get the End percent from the Battery Level. Mine always had a line for Idle (blue P symbol) after the charge. From it I get the Starting Range. I then divide the Starting Range by the End percent. For example, if the charge was to 90% and Start Range was 272.08 I'd get 272.08/0.90 = 302.3 miles. 

I am guessing they do the same for the battery degradation report. I am pretty concerned because it showed 309.8 with little variance from Jan 1 until Sep 16 and has gone down to 297 since and seems to continue to fall. I wonder if that number is based purely on voltage. I suspect it is the same as the range displaying in the car, just with more significant digits. If so, I've seen people say that number isn't an indication of true degradation. I hope so because it seems like mine is rapidly losing capacity, mostly before the temp drop.