This report describes a procedure for forecasting the Frequency of Optimum Transmission (FOT) on a high-frequency communications path when the Maximum Observable Frequency (MOF) on another path and an historical record of the correlations of MOFs measured on the two paths are known. The procedure, known as 'Updating' uses MOF values measured by an oblique-incidence sounder on one path to adjust the MOF forecast by the MINIMUF model for a second path. The model is Updated by using one of its input parameters, the solar 10.7 cm flux, as an adjustable parameter to make the predicted MOF equal the measured MOF on the sounded path. The measurement of the correlation coefficient connecting MOFs measured on a pair of sounded paths is discussed. After forecasting MOF, statistical methods are used to determine the FOT, which may be defined as the highest frequency not exceeded by the MOF ninety percent of the time. Data from the 1980 Solid Shield series of ionospheric measurements are used in examples of correlation coefficient measurements, Updated forecasts, and FOT determinations. Originator-supplied keywords include: High frequency propagation.