From Figures \ref{fig:20zsunsurf} and \ref{fig:60zsunsurf}, three trends for the extent of the sub-surface convection zones are noticeable. First, with increasing time during the main sequence evolution, these zones become more extended, and are located deeper inside the stellar envelope. This is because the stellar envelope expands, and becomes cooler, while the temperature of the opacity peak remains nearly constant. In our \(20{\,\mathrm{M}_\odot}\) model at \(Z\)=0.02, the mass of the He convective zone increases from about \(10^{-9}{\,\mathrm{M}_\odot}\) to \(2\times10^{-7}{\,\mathrm{M}_\odot}\), and that of the FeCZ is growing from \(2\times10^{-6}{\,\mathrm{M}_\odot}\) to \(10^{-4}{\,\mathrm{M}_\odot}\). For sufficiently hot models, the helium convection zones can even vanish (Fig. \ref{fig:20zsunsurf}, lower panel). Second, comparing the 20\({\,\mathrm{M}_\odot}\) and the 60\({\,\mathrm{M}_\odot}\) model at \(Z\)=0.02 demonstrates that the FeCZ becomes more prominent for higher luminosity. This is because the opacity is not substantially changing among main sequence models at the same metallicity, such that a higher luminosity renders a larger portion of the envelope convectively unstable (both in radius and mass fraction). Our models show that the FeCZ disappears below a threshold luminosity of about \(10^4{\,{\rm L}_\odot}\) on the ZAMS at solar metallicity. Third, comparing the two 20\({\,\mathrm{M}_\odot}\) models in Fig. \ref{fig:20zsunsurf} shows that the extent of the FeCZ, and its presence, depends on the metallicity. We find that for \(Z\)=0.001, it is completely absent below 40\({\,\mathrm{M}_\odot}\), and at \(Z\)=0.00001 it does not occur for \(\mathrm{M}\leq60{\,\mathrm{M}_\odot}\). In summary, our models predict an increase of the importance of the FeCZ for cooler surface temperature or lower surface gravity, for higher luminosity, and for higher metallicity.