i found this little mod put together by the author of trinity bars & trinity chat that shows a nicely colored bar displaying your effective avoidance stats and how close to uncrushable you are. when shield block is up it shows "uncrushable" and if you get crushed it turns red. i never realized how many times i dont have block up until i see it turn red during a fight.
I actually really like this little addon. It turns green when your uncrushable so its easy to see when your not. It was also surprised how often I was leaving a small time windwo with it down. hehe Very good for training yourself to keep it up as much as possible. I'm still adjusting to 2.3 I guess, much easier to hold aggro now and you do not have to worry so much about always keeping a max threat rotation going and skip one now and then to keep your ShieldBlock up.