This may be true - no one would measure a tyre to see if it's out or not.

However going by defined size ie NOMINAL tyre size, either specified on place card or vehicle specification, and size of tyres fitted to the vehicle - as stated on tyre's sidewall - would give the best method of comparing these two sizes as actual measurement would not be accurate and introduce too many other variables due to construction and tread depth and manufacturing variation of various tyre models and therefore ACTUAL measurement is irrelevant in this argument.

To me, if the specifications say 215/65R16 and the permitted oversize is 15mm extra, then the next size within the limit is 215/70R16. End of story.