Attempting to find hard and fast rules for tank gearing is impossible [imo].
You have to look at what you're trying to do [the encounter] and what problems you'll face. Do you have slightly weaker healers? What's the boss' damage like? There are a huge number of variables to take into account, and to be a good tank you have to have a good feel for them, rather than depending on a third party tool to do you thinking for you.
Only you can know what is best for you as a tank, as your situation is entirely unique.
tl;dr: Gear yourself for the encounter.