How much damage does the a piece of knights/ascalon armor reduce?

Is it 1 damage per amount of damage taken, like absorbtion runes, or is it a percentage?
In other words is having one piece really that worth while to make it a staple in every armor combination?

-2 per damage number (except life stealing)

I believe that the damage reduction replies to all damage except that which ignore armor, not just spells that steal life.