In the Civil War, the Southern states were subtracted from the calculation of an electoral vote majority because those states had been formally declared to be in rebellion. Today, no such subtractions would be made from the electoral vote majority required for the election of a President.
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.