Erice1984,
Looking at the EXCEL graph that you have posted it seems like a second degree polinomial will fit nicely. A formula of the form Y(X)=a*(X^2) + b will work. The EXCEL trendline will give a value for a and b in the above formula. Then, round this numbers up and you should have a good approximation for your graph. I am writing this reply from my cell phone so I don't have EXCEL with me at this moment to show you what it might look like.
Give it a try. If you have any problems, let us know.

Robert