By default a DK tank will want to use Rune of Stoneskin Gargoyle or Rune of Swordshattering dependent upon multiple factors. Since one of the changes in MoP is that a DK should have about 2-3 times the amount of parry to that of dodge because on separate diminishing returns, although they are on the same roll. One thing that I want to touch on before going onto the next part is that I will refer to Parry Chance percentage as the actual doll number, compared to that which I will refer to as the amount Parry is increased by.
This is an argument to say that in some cases Rune of The Fallen Crusader is more viable then the other tanking Runeforges once it Parry reaches a certain plateau. Rune of The Fallen Crusader increases the amount of strength that the DK has by 15% and heals the DK for 3% every time the buff is applied. 1 Point of strength in MoP now = .85 points of Parry. 15%•.85 = 12.75% increase in parry. Not 12.75% Parry chance. Now that we have that our of the way we can calculate how much parry a DK would need so that it is on par with the Swordshattering runeforge. To get this figure divide 4/.1275 which ends up being 31.37% Parry Chance. Which is a figure we can likely see in the next tiers, in Heroic DS I had about 47% Avoidance – 4% from Swordshattering – 5% for Miss chance and we have about 38% avoidance. Which was evenly distributed between parry and dodge, but with a 3x multipler between dodge and parry we could see 8% Dodge and 30% Parry which at that point would be pretty much a toss up for the 15% strength. After 31.37% depending on how active a player is hitting a boss using special attacks there should be a 100% uptime which equates to 4% parry 3% increased heals and 15% strength.