To answer your question, Inspiration from Priests and Ancestral Fortitude from Shamans grants a 10% dmg reduction and it stacks with reduction from armor, but not straight additively.
If your armor and passive damage reduction reduce the incoming damage by 80%, and you're seeing 10k hits, say. The 10% reduction from the proc will reduce that 10k by 10%, so to 9k hits. This is how those effects stack.
If you have 75% reduction from armor
10% reduction from stance
3% reduction from raid buff (BoSanc or Divine Grace)
and then 10% from Ancestral Fort
this is how it adds up:
1 - (1.00-0.75)*(1.00-0.10)*(1.00-0.03)*(1.00-0.10) = 0.8036 = 80.36% total