  Stellar parameters are not easily derived from M dwarf spectra, which are
dominated by complex bands of diatomic and triatomic molecules and not well
described at the line by line level by atmospheric models. M dwarf
metallicities are therefore most commonly derived through less direct
techniques. Several recent publications propose calibrations that provide the
metallicity of an M dwarf from its Ks band absolute magnitude and its V-Ks
color, but disagree at the \pm0.1 dex level. We compare these calibrations on a
sample of 23 M dwarfs, which we select as wide (> 5 arcsec) companions of F-,
G- or K- dwarfs with metallicities measured on a homogeneous scale, and which
we require to have V band photometry measured to better than \sim0.03
magnitude. We find that the Schlaufman & Laughlin (2010) calibration has lowest
offsets and residuals against our sample, and use our improved statistics to
marginally refine that calibration. With more strictly selected photometry than
in previous studies, the dispersion around the calibration is well in excess of
the [Fe/H] and photometric uncertainties. This suggests that the origin of the
remaining dispersion is astrophysical rather than observational.
