So, a lot of pe... ok no, I won't start like this.

According to SmashWiki, the formula used for knockback includes many elements, such as kb scaling, base kb, handicap, rage, weight, so on. But most importantly, it is based on the original percentage the character had when hit and the amount of damage the attack dealt.

Here's an explanation of the formula: http://www.ssbwiki.com/knockback

K = (((((p/10 + pxd/20)x200/(w+100)x1.4)+18)xs)+b)xr

So, let's try to decide the following: can we determine the new kill percentages of an attack provided we know its previous kill percentage, how much damage it did before and how much damage it deals now, given everything else stayed the same?

Yup.

(This is just a bunch of math, skip to the end for the results)

Let's clean!

K = (((((p/10 + pxd/20)x200/(w+100)x1.4)+18)xs)+b)xr

K/r =((((p/10 + pxd/20)x200/(w+100)x1.4)+18)xs)+b

K/r -b = (((p/10 + pxd/20)x200/(w+100)x1.4)+18)xs

(K/r-b)/s =((p/10 + pxd/20)x200/(w+100)x1.4)+18

(K/r - b )/s - 18 = (p/10 + pxd/20)x200/(w+100)x1.4

((K/r -b)/s - 18)/1.4 = (p/10 + pxd/20)x200/(w+100)

Now, since we said everything other than damage output remained constant, the whole left part of this is constant. Let's call it A for ease of notation.

10A(w+100) =p(d+2)

Once again, the whole left part of this is constant. So, let's say we have the original damage as d and the original kill percent as p, then the new values as d' and p' respectively. It follows than:

p(d+2) = p'(d'+2)

p'=p(d+2)/(d'+2)

BUT p is the damage AFTER the attack hit, so p = o + d, where o is the percentage char had before hit and d is damage dealt.

so this better translates to:

(o+d)(d+2) = (o'+d')(d'+2)

(o+d)(d+2)/(d'+2) - d' = o'

So, let's sat you killed a certain character with Marth bair tipper at 150% damage BEFORE hitting. That translates as 162% after hitting.

Then, the new new kill percentage is (150+12)(12+2)/(12.5+2)-12.5 = 143.9% ~ 144% nefore hitting.

about Fair tipper, let's say it killed at 150% (damage before hitting)

Now it kills at (150 + 10)(10+2)/(11.5+2) - 11.5 = 130.7 ~ 131% before hitting.