Where did he Obstruct Justice (the Administration of Justice)?
The Prosecuter sent Tate a letter saying that he would not look upon the letter to Miller as Obstruction. The US Attorney will have to prove that he corrupted her testimony in the context of the letter he sent to Tate.
I read somewhere that some of Libby's notes recovered by Fitz were on a hard drive that Libby thought he had wiped clean. That would be enough for an obstruction indictment. I doubt it comes from his letter to Miller.