Say you've got a Killing Attack. The ratio of KA to DC points is 5 / 15, or 1/3.
1 DC = +1, 2 DC = 1/2d6, 3 DC = 1d6. Not a problem.
But then it gets an advantage that affects how it does damage for +1/4. It's now 5 / 18.75 = 0.2667, or 0.25 or 1/4d6.
How do I convert that into +1, 1/2d6, 1d6 and so forth? I've read the rules explanation, but something isn't sticking.