Ionization fraction versus integrated line intensity ratio for the best six ratios found in Sect. 4 for translucent medium conditions. The chemical model grid is shown as a scatter plot, with the central crowded regions replaced by PDF isocontours containing 25, 50, and 75% of the points. Superimposed are the RF model (red line), the analytical fit (solid black line), and the analytical fit of the 1σ uncertainty (dashed black lines). The quality estimates of the two models are indicated on the figure.