I highly doubt perfection of mitigation will be a requirement, likely it will more so be a thing that has a sizable margin for error, that is what it will be balanced around.
I great tank when it comes to mitigating things will be able to go above the expectations, where likely content will be balanced around a mediocre player, not a pro player or a bad one.
Bad players will likely not be hit very hard, at least no more than threat already has wasted them on, mediocre and highly skilled ones will have something more dynamic and or to work to.
I think the goal being, what is "fun" is appropriate. There will be always good amount of lead way when you design content around player skill versus raw data/numbers (like dps spec rotations to gear ratios and such).