Cumulative distribution function of the line-to-line scatter estimation of the derived [Mg/Fe] abundances per spectrum, for weak non-saturated Mg lines (top; 4730.04, 5711.09, 6318.7, 6319.24, and 6319.49 Å) and strong saturated ones (bottom; 5167.3, 5172.7, 5183.6, and 5528.4 Å) separately. The blue curve describes the values that correspond to the normalisation interval that presents the lowest χ² fitting value, the red one corresponds to the continuum placement performance in the typical wavelength interval used in previous works in the literature, and the black curve corresponds to a narrower local normalisation interval around each line as proposed in this work.