Most everyone who's responded to this thread has the right idea. No one tanks in a vacuum. There's a lot more to do when tanking raid bosses besides soak up or avoid damage. For fights like Lady Deathwhisper, Putricide, the Blood Princes, Valthiria, Sindragosa, and the Lich King, there's a ton of movement involved. But there's no equation that'll evaluate how well a player moves or reacts.
I think we all understand the desire to be able to evaluate someone before we invite them to a pug, but there's really no way to quantify everything a player is, and certainly not something like skill and situational awareness. This borders on trying to remove the human element from the game, and that would be a horrible thing.