The problem with calculating mitigation in this manner is that the actual would tend to be widely variant and per situation and fight based. It doesn't take into account player skill or knowledge and views these classes completely in a vacuum which negates natural synergy with other classes and per fight mechanics. For instance if two Pallys enter their own instance with the same gear and both MT the fight, the way cooldowns are used, team strategies, and just general skill will have varying results in terms of mitigation as a whole making the above wholly unreliable as a means of objective assessment.

Also mathematically speaking this assessment falls short for a few other reasons, the first one being user differential in using cool downs (people don't just blow cooldowns because they come available, rather they tend to use them when it is advantageous in mitigating larger amounts of damage or saving them when necessary making the cooldown time assessment unreliable as a mean of mitigation measurement.) It doesn't account for tank stance which increases mitigation base for pally and drk while increasing parry and hp for warrior. (ehp) It isn't properly accounting for the ultimate cooldowns (which can't be judged within the limited context of these calculations, yet any damage acquired at "1hp" while under the affects of holmgang or living dead is 100% mitigated and depending on the fight this could vary widely with outgoing damage and cooldown usage.) Whether or not foresight is available to all tanks or not it should be considered in the grand scheme of things as the warriors foresight is superior than on other tanks. This also doesn't account for parry/block nor does it account for healer synergy created by things like convalescence which while not mitigation wholly increases survivability of the tank. Last but not least it doesn't account for raid wide mitigation (which would be per fight and is utility) like for instance in final coil, dragon kick (or delirium blade now) was/is one of the highest raid wide mitigation abilities in the game due to the prevalence of magic wide aoe attacks in high level content.

TLR I don't think this is an adequate means of assessing tank mitigation and such things are more emphasized by the particular fight mechanics/boss damage/player ability than just analyzing tank cooldowns in a vacuum. Analyzing in a vacuum is nice when looking at certain things but when trying to assess actuals it comes up extremely short. Player ability should not be understated as active mittgation is 100% player reliant.