My current calculations is at 800 Defense/Mres you basically have about 34% damage reduction from those types of attacks, then the question is how other effects add to that and in what order, and how you can calculate your total damage mitigation.

Assuming that Shield Oath, Ramparts and Sentinel "stack" in the addative way that it makes a grand total of 80% reduction, but since we already have 34% mitigation from our armor we are still taking damage even though the total mitigation added should be at 114%, from that we can assume that the armor mitigation happens at another step of the damage cycle.

I guess it takes the "magical" reductions first such as Shield Oath, Ramparts and Sentinel first, and then apply the rest of the damage on the armor before the final damage taken is calculated.