"Equal temperament is NOT based on C and can't have anything to do with it."
"Pianos are never really tuned to equal temperament. Look up "stretch tuning", "

Well the piano tuner who visited us in the 1960s claimed it was 'equal', but he did it by hand, counting beats and 'stretching'... and he DID it in C.... :-)



You see the real answer to the original question partly depends on what instrument....