https://doi.org/10.1351/goldbook.R05419
The ratio: \[f_{\text{r}}=\frac{A_{\text{r}}}{A_{\text{g}}}\] where \(A_{\text{r}}\) is the @[email protected] (true, actual) @[email protected] and \(A_{\text{g}}\) is the @[email protected]