Overall, your gear looks fine. I don't agree with the onyxia trinket though, and as I recently found out Stamina + armor or armor + avoidance trinkets are indeed the way to go, despite the item level.
To answer your question, there is no ideal avoidance over all that you need. Obviously 102.4% was the avoidance cap in BC needed to not be hit with Crushing Blows, but since these are gone now it is not needed. On top of that, dodge + parry ratings have the same diminishing returns now, and avoidance stacking is now frowned upon due to this, since you don't want healers to be healing you, then see your health almost go out completely in a matter of seconds.
There are indeed fights where avoidance may be a good thing, but over all I'd say just keep avoidance you get with the gear, concentrate on armor + health and aim for that.
Also just looking over your glyphs and talents, if you want just for this fight to take less damage, I'd suggest changing glyphs around, using last stand and Enraged Regeneration together with other health bonuses, and popping that shield wall with a trinket when you know you will take heavy damage.
I do not have any math or statistics to put in here to prove my point, but trust my word. I have taken that advice, and healers have applauded the results when I tank in raids. Look at my things if you'd like, ignore my trinket selection and tier 10 though, found out they aren't the best (Except for the Indom. trinket).