If Annabeth had an Achilles where would it be? [General]

As I said, if Annabeth had and Achilles spot like Percy where would it be? Not going to into detail, but Percy got his after bathing in the river Styx. Let's say Annabeth bathed in the river Styx instead, where would her Achilles be? Percy's is the small of his back, I think Annabeth would go for a easier to protect spot but that still leaves the question of, where?