First off, this skill stacks up to 5 times. End result, +1000 parry rating, 3000 damage on next parry.
Now, the trick for DK's with this for DW is the fact that it can proc off all of your normal white weapon swings, your plague strike swings for both hands, your blood strike swings for both hands, your obliterates for both hands, your frost strikes for both hands, as well as your rune strike. So, unlike a warrior or paladin who only should expect to see an average of 1-2 with a high of 3-4, a DW DK can expect to see an average of 2-3, a well as a significant increase in your single target threat on top of things.
I do use the simple avoidance meter i got from curse called...... "avoidance", and I track my progress. Going in with DW Legacy of Thunders, I have 27.63% dodge, 18.04% parry with horn. My total avoidance at base reads at 56.60%.
Now, I do have the new Sigil (even tho it says chance, it has a redic procrate and is almost always up. This in essense is what allows DW DK's to stack so much more of the BW stacks. When the sigil procs in a raid setting, I usually have about 31% dodge, so I have a much higher chance to dodge an attack than parry it, meaning that my stacks don't all get wiped constantly like a warrior or paladin who have much closer dodge and parry numbers. Usually about the time I start getting to 24% parry, (meaning I am now at over 64% total avoidance) is when the stacks will either fade and start over, or the proc will hit.
My other benefit comes in the form of my pocket healer, a resto druid, who has that lovely talent on Wild Growth that gives me free RP. Can anyone say RS and FS out the ass?
So there you go. its late and im tired. bask in the glory that is the blade ward enchant, just make sure you have a crapton of EXPT