The logic is if the ball is too far back it starts moving forwards without twist then there is a transition from no twist to twist. If the ball is well forward it goes straight to twist.
Its a bit like a cartridge round when you load your own you seat the bullet nice and close to the rifling so it picks it up straight away.
Nothing in muzzle loading is hard and fast, what suits one shooter or gun may not suit another, part or the experience is the experimentation.
Myself I use couscous as filler and seat the ball as far forwards as I can. Its given me a massive improvement in my shots. I'm now at the point where I am hyper critical with my powder measuring and weighing my balls but funily not getting a noticable difference.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane