Results of fitting the dependence of γ on ${\overline{x}}_{\mathrm{HI}}$ using Eq. (5). Top panel shows the best-fit values of the parameters log γ₁, γ_c, and γ₀ when we fit different curves in the middle panel of Fig. 5 using Eq. (5). The bottom panel shows the difference between the input or true γ values and predicted/fitted γ_f values with the best-fit parameters as a function of average ionization fraction ${\overline{x}}_{\mathrm{HI}}$.