It's not that difficult. It just takes time. You require a professionally calilbrated ECM8000. It would be adding extra variables if you used a generic ECM calibration file.
The method is to position the calibrated microphone and the unit under test quite close together to eliminate errors caused by positional differences. Then, using a subwoofer, you sweep from 10Hz to 200Hz and record the readings. It's important to use a system with as flat a response as possible to obtain the smoothest graphs so as to avoid rapid changes in response that produce anomolies.
The resulting mdat files are converted to text and comparisons made as to the differences. The calibration file is derived from those differences. Once the file is created it is then tested to ensure both the ECM and unit under test match sweep to sweep...
Be cautioned that using measured data (as we have done) is a dubious method to derive a calibration file for a meter. We simply decided that it would be a heck of a lot more accurate than what was available at the time. Aside from the questionable method of deriving the files, the discrepancies between like units is completely ignored. You would really need a better sample size to derive any real confidence in the results.
The easiest thing to do of course is to simply purchase an ECM8000 and preamp. They are quite accurate and ridiculously cheap....
brucek
look in
this thread for the generic ecm8000 file