when x is 40%, i am not getting y=10%

That's not equal percentage then. for 40% increase in x, y should also increase by 40% for an equal percentage curve (as per my understanding).

I am guessing you are looking for modified parabolic or hyperbolic, where small increase in y leads to greater increase in x.

y=K.exp(mx)+c

I don't have access to Excel or Matlab right now to verify. Try to apply yourself. Try playing around with K to get the percentage change. I am guessing, it should be 1/4 in your case assuming m to be 40.