I was thinking about the gear ratio method and how 100%MM and vertical 100%MM follow the same principle, and yet they use different monitor match percentages... and then it clicked - the point on the screen you match IS the "'edge of the monitor". Therefore, using vertical 100%MM is just as arbitrary as every other percentage, even if it is aspect-ratio independent.
Every monitor match percentage is "synced", as per the gear ratio concept, it just changes the size of the window you look through.