I've been wondering the very same thing myself for the cab mod I'm doing. After spending time at a nearby arcade with a couple pinball machines and looking at my X-Arcade dual joystick, the only conclusion I've come to is that pinball buttons should go wherever it feels comfortable for your index and middle figer when you grip the sides/corners of your cab/CP. I did measure the pinball buttons distance from the top and side, but I soon realized that those measurements would do me no good, since my CP/cabinet do not have "box corners" like a pinball machine does.