They chose an odd scale for this unit, it looks more like a digital scale with 0dBFS being the top. Most analogue VU meters will go beyond 0dB, so not sure what this unit is doing.
The normal procedure for calibrating a meter is to use a reference voltage, the reference would usually be a tone of say 1khz and with a voltage of 0.775v. You would then measure the voltage at the output and trim the VU meter accordingly.
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