Comparison of synthetic spectra around the saturated line 5183.6 Å (black solid vertical line), considering all the absorption lines (blue spectra) and only the Mg line absorption (red spectra). The black and red dashed vertical lines present the limits of the 6 Å and 15 Å normalisation windows, respectively. Left: hot star (T_eff = 6000 K). Right: cool star (T_eff = 5000 K). The other atmospheric parameters are constant (log g = 4.5 dex, [M/H] = +0.25 dex, [α/Fe] = 0.0 dex).