A mutual fund company has the following fee schedule:

On 50 miillion they charge: 0.75%
On 100 million they charge: 0.75%
On 200 million they charge: 0.70%
On 300 million they charge: 0.683%
On 400 million they charge: 0.675%
On 500 million they charge: 0.67%

I am trying to come up with a line that best fits. I can't do a linear regression since the data is nonlinear nor can I use the growth or linest functions in ms excel because they are for exponential growth.

What functions in excel can I use to come up with a nonlinear equation that best fits the data so that I can forecast charges for money on say 750 million dollars??? Also, can I use this equation to forecast charges on money below 50 million dollars?

Any feedback would be greatly appreciated.