Since the Constitution does not set a number, it’s common sense (and I’m sure legal precedent) that you vote with what brung ya, or the majority that shows up. There can be, and has been, no precedent for a “quorum.” Hence, the Civil War example remains the example.
When all the marbles are at stake, common sense and historical precedent are weak considerations easily brushed aside on the way to a preferred result. For clever lawyers and an accommodating Supreme Court, the Twelfth Amendment could provide a counter example and plausible authority for a two thirds quorum and a majority vote of the states to chose a President.