Just so I am clear about the equation, I have re-written it with TeX:
It can be re-arranged very slightly:
but it doesn't help very much.
If you only had to find a best fit for one variable, I would have suggested using a Linear (or Non-Linear) Least Squares data-fitting routine, depending on the curve described by the data. However, I have never seen it done for two variables. Perhaps MATLAB or some other commercial software package has these capabilities which I don't know about. Do you have access to MATLAB or a similar package?
Alternately, perhaps you could assume an exact equation, pick two points, and simply solve for the equation at those points (two equations in two unknowns).
Not sure what else to suggest.