the damage reflected is a % of the damage taken. Blazing/Freezing Skin lvl 3, Chant of Revenge lvl 3, and Song of Vengeance all do 20% (or 1/5) damage back. However Song of Vengeance can stack on any of the other 3 for a total of roughly 33% (or 1/3). I'm pretty sure it all stacks on armor bonuses as well and I believe the one cat's passive as well.
Reflect damage is awesome for raid bosses, raid bosses can hit tanks for anywhere between 300-900 so you can reflect at least 100-300 damage a swing. Its a small way to offset the terrible XP a tank group gets at raids.
However about Dark Avengers reflect damage, I don't know. I assume its just like the other reflect damage buffs.
ohh now I know what you mean. You mean like if you smacked a person with like 1 pdef and do 10K damage will 3.3K damage be reflected? I don't know but i would assume so. Would be a fun test to try out in the future :)