HF propagation prediction is a statistical science.
When a sporadic Es layer is present, its maximum usable frequency relates to its ordinary critical frequency approximatively as follows:1
\[MU\!F_{E_s} \approx \frac{f_{o,E_s}}{0.188} \approx 5.319\,f_{o,E_s}\]
For the real-time ordinary & extraordinary wave critical frequency of the F2 region (fo,F2 & fx,I), the height of sporadic Es blanketing or screening (hEs) and the real-time the maximum useable frequency (MUF):
For the real-time effective solar sunspot number (SSNe):