Contour plots where the third dimension are iso-flux lines, in particular for the fr_CaHK/Hβ flux ratio. Those iso-fr_CaHK/Hβ lines are temperature-sensitive up to about [Fe/H] = −3.5 dex, and then they become metallicity-sensitive, hence we can use them to find a relation from which we can infer [Fe/H]. Because the metallicity sensitivity starts at [Fe/H] = −3.5, we cannot distinguish iron abundances below that threshold, but rather identify whether they are below or above it.