A logarithmic function of the \(R_{\rm{F}}\) value (retardation factor): \[R_{\rm{M}}=\log _{10}(\frac{1\,-\,R_{\rm{F}}}{R_{\rm{F}}})=\log _{10}(\frac{1}{R_{\rm{F}}}\,-\,1)\]
Source: PAC, 1993, 65, 819. (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 845 [Terms] [Paper]