The balance pot is there to make small corrections for imbalances that may exist due, primarily, to mistracking pots but also small differences in tube gain between channels, level differences between channels in a source component, etc. So there is no arbitrary universal mark that could be applied to the panel. As a starting point one could play around with a split mono input signal and a meter on the outputs to find a level match between channels as PB suggests. But that will only be balanced at the mid level volume knob setting. Balance could be different at different volume levels, particularly very low ones. And even that does not take into account differences in sensitivity of one's ears, differences in speaker sensitivity, etc.
Bottom line, the best way to find balance is to listen and adjust to taste.
