Illustration of the fit to time-average values of ${\xi }_{\mathrm{ion}}^{\ast }$/${\xi }_{\mathrm{ion}}^{\mathrm{HII}}$ at ages of less than 1 Myr as a function of the metallicity provided by Eq. (8). The time-averaged values are estimated from Fig. 7. Approximate uncertainties are calculated as the distance between the most extreme point and the mean value at each metallicity (red line) in Fig. 7, and are accounted for in the estimation of the best-fitting relation.