A logarithmic function of the RF value (retardation factor): \[R_{\text{M}}=\log _{10}(\frac{1\,-\,R_{\text{F}}}{R_{\text{F}}})=\log _{10}(\frac{1}{R_{\text{F}}}\,-\,1)\]
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 845 (https://doi.org/10.1351/pac199365040819)