The formula is y/100=K^(x/100-1). K can be any constant which will change the shape/rate of the curve. Here is a check with K=20.

Here is example for 10% increase.
x                 y            percentage increase
0                 5.00             25.89
10               6.75             25.89
20               9.10             25.89
30              12.28             25.89
40              16.57             25.89
50              22.36             25.89
60              30.17             25.89
70              40.71             25.89
80              54.93             25.89
90              74.11             25.89
100            100.00           25.89

Here is example for 15% increase.
x                 y            percentage increase
0                  5.0                   36.2
15                7.83                  36.2
30               12.28                  36.2
45               19.25                  36.2
60               30.17                  36.2
75               47.29                  36.2
90               74.11                  36.2