I had this same question too when i got around 490 defense. Here are some things that I noticed as I have been progressing and don't directly answer you question but will give you something to think about as you are gearing up. (My warrior's name is Beatitude)
1. Rage = threat
You need to be able to generate rage if you want to be able to hold a target. The ways a prot warrior generates most of this is through getting hit. Although high dodge+parry is great for not taking damage, they don't generate rage. Sometimes you need to take a hit or two to get the rage in necessary to establish and maintain . This is why I am learning to like block rating and block value more and more. This allows you to mitigate some damage while still being able to get rage. Be careful how much dodge and parry you stack, it may lead to some threat problems.
2. Stamina, Stamina, Stamina
You can never have enough HP. If you have the choice between throwing a 12stam gem into a slot or a 6+stam + something else to get a small dodge or block bonus, go for the 12 stam. I don't know if this holds into BT or MH, but it sure as helps in instances/raids before those at that level.
3. 490 def is a minimum
I usually see most warriors with between 500 and 520 defense. Defense is a nice stat in itself because it raises dodge, parry and block. Over 520 defense there seems to be diminishing benefits, which is why warriors tend to go toward +hit rating, expertise, and attack power. Which brings up another point.
4. Try not to miss
After awhile you might notice you miss bosses way too much. This can be supplemented by hit rating, expertise, and attack power.
I stole a lot of that from guides I read about being a warrior, but it is worth mentioning. If you got something else to add or just want to make fun of me, that is cool too.