When you buy a mic for doing measurements they supply a calibration file for it. With the UMIK-1 for example you go on their website and enter the serial number of your particular mic and download the calibration file. Each mic is tested against a reference before it leaves the factory. Hassle with the mic that comes with your amp is you probably don’t have the calibration file - I guess it is baked into the amp (if they even calibrate them individually).