I suggest this:
D Rank - 10%
C Rank - 15%
B Rank - 20%
A Rank - 25%
S Rank - 30%
SS Rank - 40%
X Rank - 50%
Not only would this make things a lot more fair and balanced, but it would allow for apps to be graded on a scale and help out graders by setting standards for everything instead of trying to balance it by itself.