You'll get it by applying statistical caculation rules, if your application actually requires to determine a correlation coefficient. I never came across such an application, neither I'm aware of a definition of "C" in electronics text books. I understand it's meaning similar to the
concordance correlation coefficient discussed here
https://en.wikipedia.org/wiki/Concordance_correlation_coefficient.
What's your intended usage of the correlation coefficient? It's not present in the standard transistor models used by circuit simulators. I already explained why I expect a low value of correlation coefficient for BJT.