It is strongly recommended to use a calibrator for checking the level reading (and sensitivity) of your measurement device.
For measurements with microphone, quality sound calibrators like Nor1255 and Nor1256 are available.
If both the calibrator and the complete Nor145 (including microphone and preamplifier) have been sent to an accredited
laboratory like NCL you should have correct values to use for the calibration check.
Calibrating Microphones