The maths itself is correct; but the Mastery values are (in actuality) off as you're not factoring in Mastery buff from the raid. Doesn't make a huge difference to the results, though.
Without wishing to be rude, here, and I certainly don't
intend to be, but I would take ranks this (relatively) early on in a tier with a pinch of salt; it's not hard to get fed gear in a 25M setting (which, having checked your Armory at the time of your ranks, you were) and then blast parses out early. I wouldn't use them as a credential, in all honesty.
The one mistake you're making (and I see made often; I myself used to make it) is that you think Mage Armor adds 10% damage because it's 10% Mastery.
10% Mastery != 10% damage, and it's actually pretty easy to show you why:
Say you're at 10K Mastery; not exactly "high" for this Tier. Raid buffed you'll be at 60% Mastery. With Mage Armor, you'd be at 70% Mastery.
Assuming you're able to stay at 100% Mana, with FA your multiplier would be 1.6, with MA 1.7
Code:
100*1.6 = 160
100*1.7 = 170
1-(160/170) = 0.0588
So in reality, you're "gaining" 5.9% damage
when you're at 100% mana, despite the fact you're gaining 10% Mastery.
This trend goes on as you get more and more gear:
Code:
100*2 = 200
100*1.9 = 190
1-(190/200) = 0.05
People need to understand that Mastery suffers from diminishing returns. Sure, Haste has diminishing returns too, especially when you're getting close to GCD-cap, but people need to stop thinking that "10% Mastery = 10% Damage".
The
reality is that due to various reasons (such as Mana return, DoT ticks and more casts throughout a fight), Haste and Mastery are roughly equal in terms of stat weight. Therefore, due to the gaining returns you get from Frost Armor, I argue that it's better purely because you're gaining more "stat value". That's just my argument.