You need a total of 17% hit in order to avoid any miss on boss. That is since 3.0 where they removed the 1% limit.

365~ hit (14%) without SP/Moonkin in raid
285~ hit (11%) with SP/Moonkin.

263 if you're a blueberry.


Also more info here:
http://elitistjerks.com/f31/t29453-c...gs_level_80_a/