Well i guess Test records are perfect for this (with an accompanying osciloscope or ADC).I expect that is the manufacturing tolerance and the value will differ from unit to unit. How will you know your cartridge’s balance level and how can you then compensate such a small difference using a much wider range balance control?
Finally, why was not that a thing when we only listened to vinyl?
PS. I’m trying to learn, not rebuke.
My Cambridge audio duo has a knob spanning -4 to +4 dB with like 270 degrees of movement. Sure it won’t be digital accurate but with some patience I guess you can get pretty close.
I know for sure that my Ortofon 2M blue has audible channel imbalance, it’s not the most expensive stylus but it’s not the worst either.