Left: obtained abundance ratios [C/Fe] vs. [Fe/H] of the X-shooter catalogue with the BOSZ synthetic spectrum grid (Mészáros et al. 2024). The red points are defined as in Fig. 3. Middle: direct comparison between the derived [C/Fe] abundances with each reference synthetic grid, colour-coded according to the stellar metallicity. The dashed black line and the offset estimates are the same as described in Fig. 5. Right: difference in the measured [C/Fe] abundance between the grids, colour-coded by the effective temperature, as a function of the [C/Fe] estimate previously determined with the grid from Knowles et al. (2021).