Nevermind got the solution myself.
While looking at mathematical functions and graphs I found the power function, which seemed perfectly for my problem. Now I needed to get it to cross my two points P(0|-25) and Q(100|0).
I started with the normal formula y = a * xn. Using 4 as n would make the curve start very late and make that nice stop effect at the beginning of every movement.
Now inserting the two known constants, I get: 25 = a * 1004. Reforming that sentence gives me a = 0.00000025.
With that I was able to recreate my formula and use it in my game:
y = S / D4 * x4 - S