So the top of the graph says, "Level Delta". Delta is a Greek letter which is used to represent "Difference" in science and math. So the Level "Difference" between a 300 and a 320 is 20. If you check the graph and see where the Red "Taken King" line reaches 20 on the x axis you can see the Y axis is at ".10". That .10 represents "Damage Reduction." So multiply the damage usually done by .10, example 36 X .10 = 3.6. 3.6 is 10% of 36. Therefore if a 320 meets a 300 in Iron Banner, the 300 will be doing 10% of their normal damage.