Last time I looked Tankpoints was spot on with its calculations the problem and why everyone stopped using it was because its based around a melee boss like patchwerk because that was it's vintage.
You can't mitigate or avoid what isnt avoidable such as magic/specials etc damage.
The whole "71 sb = 616 armor" question is nowdays defunct because you cant block what isn't blockable and there in lies the problem with the addon in the current game.
As Fledern said I am sure the maths behind it was correct for a melee only boss .. I can't say if it still is I don't know and there have been alot of tweaks to wow.
So the basic problem with the addon is it gives green +rating to gear thats only going to work on a melee only boss.
If you could turn the mitigation calcs off and run it simply on EHP which it shows it would probably still be in use but you can't.
** Remember Warcraft players fail in directions you never thought possible.