Well, hover over your charscreen AR and check your chance to hit after fighting a "common enemy" you're farming (e.g. fauzt or some bosses).
Personally i used ~33k AR for farming (thats 95% in fauzt) and almost 50k against bosses (thats 75-90% against Astrogha/Xazax/Belial etc.).
Trying to kill a boss with 50% cth is pointless compared to less damage and 85% cth.