Yeah, you got it right.

  But, just a little note here.

  She is good AT grammar  (generally speaking, that is, by my experience and having noticed that she is)

  "Josi is good AT grammar."

  I can see that she is good, I have read her texts and never (or almost never) I have seen a mistake.   Yet, there´s no mark, no text, no exam involved, I have never seen her grades to make this statetement.

   "Josi is good IN Grammar."

   Yeah, she is a sophomore student, and she is good IN Grammar, Maths and Philosophy, she gets A´s in all those subjects.   On the other hand, she bad IN Physics, her grades are always C´s.