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It seems like Avio are out of business now? CAlibrating with a 10kg weight will not be accurate at all. A home gym weight could easily have 2% error, you are only calibrating in one orientation (horizontal crank), you are calibrating with around 10% of your expected values so any error in calibration will be magnified at higher forces... I wouldn't trust that device to give accurate power numbers.
I don't know what powermeter you are referring to, but crank based powermeters work by measuring strain in the crank arm. When you push on a pedal, you are notjust bending the crank arm in the main plane of the bike, but you are also twisting it. There is a chance that the powermeter manufacturer has created a stiffness profile of the crank in question and loaded that into the firmware of the device.
What is the calibration method they suggest? If they don't know the stiffness of the crank you are mounting it on, it will be quite a difficult thing to do at home.