I would say where your bone ends. The curve, if I’m understanding correctly, would be a bit too far out, and if you have a padded, constructed shoulder with it terminating at that point, it’d probably look slightly too big.
As for button stance, I sort of addressed this earlier, but you probably want the button somewhat near your navel. Now, if you’re talking about the distance between your stomach and the fabric of the jacket itself (ie: parallel planes of the jacket and stomach), I’d says around 1” or so? It really just depends how much you like the waist suppressed on your jacket. I prefer it to be moderately trim, however, I like to make sure I can still move around and that the fabric isn’t “pinching” at the waist and stretched to the point it causes ripples.
This should help.