Yes, you can simply extrapolate your rated range from say, an 80%, or 90% charge, to 100%, and subtract that from the rated value when new, which for you was 270 rated miles. Or divide the extrapolated 100% value by 270 to get a percent degradation.I'm assuming to calculate degradation as a percentage is simply converting from percentage to mile on the cars range? Simple math then to subtract the miles noted today from the new car capacity provided when it was new? Otherwise how would one calculate degradation of the battery? I have a 2015 85D with about 32,000 miles.
To minimize rounding errors, it's best to extrapolate from a high percentage charge.