I am currently working with data from a physics experiment, and I just made power regression. I get a function from it, something like
Formula: copy to clipboard 
f(x)=1,00012x2,9998

The coefficient of determination is of course 1 because it is power regression, but what I really want is to know how well my data fits the specific function
Formula: copy to clipboard 
f(x)=x3

Is there a way of making Excel find the coefficient of determination for how well my data fits that specific model?

English is not my first language, please excuse any errors.