I'm a 10 man raider , forgot to mention that.
The reason I have a higher dodge than parry ratio is due to gift of the wild and my trinket. I'm seeing about a 1% increase to parry from Gift and IIRC about the same from the trinket, that's a 1 min CD. I'm also noticing the same trend from the top tanks I watch (with dodge being higher than parry by a % or 2), the only conclusion I could come to is the reasoning I used.
I plan on the Mastery with Dodge use vendor trinket, paired with the Stam and proc Mastery on chance.
Based on your math it would seem that going for more avoidance after CTC cap would yield more net gain, albeit a small one.
There is something so appealing about backhanding someone across the face with a shield.