If your idle timing was optimized you might find that the engine likes 25/35 degrees at idle. If you set it there, with a typical stall convertor, the engine will bang the trans hard on engagement.
Well then its not optimized. :poke:
As you point out.
That's just the old fallacy that advancing until maximum rpm in neutral is useful for something.
It doesn't work for the same reason timing for other throttle positions isn't set in neutral.
We know from experience 18 is about right for those cid and duration.