How do tank mitigations stack?

[VictorSpoils]

Further more, this is why it can be better to stack different types of mitigation.
For example a shield (TBN) with a damage reduction (Rampart) will actually complement each other, as you're reducing the amount of damage being taken, which allows the shield to cover more of that damage.

e.g. You have 1000 HP, you use TBN for a 250 HP shield, and Rampart for a 20% damage reduction. This allows the TBN to cover 312 raw damage.

Probably worth mentioning that this relies on the shield still getting consumed fully. Otherwise it backfires.