Warrior Tank - Dodge vs Parry

With current mechanics, I see every reason to reforge dodge into mastery > parry > threat stats but I see posts here saying parry should be only be 2.5% greater than dodge to get maximum benefit from Hold the Line (HtL). What information determines that percentage and how is the talent mechanics such that this would be the case?

In the Rawr app, I see it recommends reforging some parry to dodge as well (although I see the model status is "Mostly" ready for Cata)

Since dodge and parry are subject to the same diminishing returns, why would you ever choose dodge over parry? I get that some dodge enchants are just better for certain slots where there is no better alternative, but I see no reason to ever choose dodge over mastery or parry.

School me.

Warrior Tank - Dodge vs Parry

Fictional example to illustrate:

If you have 5000 ratings points...

5000 dodge = 25% dodge

5000 parry = 25% parry because the dr is the same for each if you have the same amount of ratings points in each. The more you add, the more you lose to dr.

The catch is all 5000 in one or the other is much deeper into dr so...

2500 dodge + 2500 parry = 14% of each. Total of 28% avoid with the same amount of ratings. You are only half as deep into the dr.

A perfect balance will get the most combined effective avoidance because you are losing the least to dr.

For warrior and HtL, however, crit block changes the equation. If you search this site a bit, there is a graph running around that shows you how much higher parry can be and still get more damage reduction despite losing parry to dr.