We learn in the movie that Shanks had a bounty of 1 billion 12 years ago, it matches with his fights against Mihawk so we can assume that Mihawk had the level of a YC1 12 years ago
We don't know when Mihawk obtained the title of best swordsman + his shishibukai title but one thing is certain is that in 12 years, Mihawk has become even more powerful than before
