Calibrated MC values using the KL22 photometric metallicity grid and using NH22. The left y-axis shows the [Fe/H], and the right y-axis shows the [M/H]. We calculate each point along our metallicity grid lines with the KL22 grid and calculate the average [Fe/H] for each MC. The average values are shown as the gray circles, with a best-fit MC-[Fe/H] calibration plotted as a dashed black line and described by Equation (4). The MC–[M/H] calibration from NH22 is provided as red squares, with the best-fit line to this calibration shown as a red dotted line. There is an offset between the two calibrations of ∼ −0.5 to −0.75 dex for the metal-poor MCs. This offset may be a result of α-enhancement in the subdwarfs or from the systematic offset applied by KL22 to M subdwarf metallicities to account for differences in [Fe/H] measurements of M subdwarfs in wide-binary systems with FGK subdwarfs.