I think you misread the numbers, top numbers are how much it currently takes with the patch, the bottom numbers are your proposal, so let's say prepatch you needed 7 body shots to exactly kill somebody. With the way things currently work, it takes 11, with your proposal, it would take 9 [so if you're an archer, you'd save 2 arrows with your suggestion].
Math:
We have damage=x*d. x is the number of projectiles, d is the damage per projectile.
h=x*d, h is health [with my assumption of no overkilling]
If we lower d by 2/3 then we have
h=y*d*2/3, y is the new number of shots
3h/2=y*d=3xd/2
So y=3/2*x [in other words, you need 50% more shots].
Same logic applies for the 75%, in that case you need 33% more shots. Then I just go to the highest integer.
For my builds, 15/24 with 0 IF, I'd die in typically 1 hit [unless I got lucky]. For my current build 21/15 [6IF, I think], I typically get killed by a high strength character using a heavy weapon. A katana is normally 2, sometimes 3 shots. 1hers can be 1-4ish [I don't use helmets either].