On my rifle, behind the entry thimble is about 1/4, which is too thick I've been told. Eh, newbie mistake.
As far as what it should be. I don't know. I saw a nice original rifle that had amazing architecture. The lower forestock was egg shaped, and then about a half-inch to an inch behind the entry thimble it transitioned to to a V shaped upper forestock. The transition was amazing. That was when I truly learned what "architecture" meant when applied to longrifles. The rifle was early, around 1770-80, as dated by the owner.
So, I would say that its however you want to shape it. Whatever feels right. The tansition is also important.