CZR for different metallicity measurement approaches (listed in Table A.1) based on total metallicities (top) and iron metallicities (bottom). From left to right: CZR using the E-MILES library with age constraint ≥8 Gyr (method A), scaled-solar MILES models (method B), α-enhanced MILES models (method C), E-MILES models without age constraint (method D), and line-strength indices (method E). The grey symbols show the points from method A as reference, and the lines show the respective fits. The best-fitting parameters are found in Table A.2.