https://doi.org/10.1351/goldbook.15314
For a solute \(\ce{B}\) in a system containing solvent \(\ce{A}\) and a reference solvent \(\ce{C}\), \[m_{\ce{B}}^{(\ce{C})} = m_{\ce{B}} \overline M/M_{\ce{C}}\] where \(m_{\ce{B}}\) is the molality of \(\ce{B}\), \(M_{\ce{C}}\) is the molar mass of a reference component, and \(\overline{M}\) is the average molar mass of a mixed solvent, defined for a system containing two solvents as \[\overline M = x_{\nu,\ce{A}} M_{\ce{A}} + (1 - x_{\nu,\ce{A}}) M_{\ce{C}}\] the solvent amount fraction of solvent \(\ce{A}\).
Note: Used most frequently in discussing comparative solubilities in water (\(\ce{C}\)) and heavy water (\(\ce{A}\)) and their mixtures, but applies to any reference component, where solvomolality is a more appropriate term.