If I take the log of my concetration data， I can easily fit a linear regression to it and I get good R-square values.

However I suspect R-square is not telling me what I want to know: I want to know how good the fit of the curve to the actual data is.

R-square gets smaller, the smaller the slope of the fitted line is, even when it looks like it is actually quite a good fit.

How else can I describe goodness of fit for a linear curve fit when the slope of the line is very small?

thanks for your help