The maximum centre-of-mass energy of the FCC-ee is yet unknown. It was inferred in Ref.~\cite{Bicer_2014} that, if the total RF voltage were increased by a factor 3 with respect to the baseline, a centre-of-mass energy of 500\,GeV could be reached, and an integrated luminosity of $500\, {\rm fb}^{-1}$ could be delivered over a period of three years, as displayed in Fig.~\ref{fig:lumi} with the red dashed line. In the framework of the FCC, however, the interest of such an upgrade could not be demonstrated for the physics of the Higgs boson~\cite{Bicer_2014}. It is interesting to re-evaluate this statement in view of the physics of the top quark.

The centre-of-mass energy was therefore varied from 350 to 500\,GeV, and the corresponding integrated luminosity was varied linearly with $\sqrt{s}$ from 2.6 to 0.5\,${\rm ab}^{-1}$. The expected uncertainties on the top electroweak form factors, $\sigma(F_{1V}^\gamma)$, $\sigma(F_{1V}^Z)$, $\sigma(F_{1A}^Z)$, $\sigma(F_{2V}^\gamma)$ and $\sigma(F_{2V}^Z)$, were determined as explained in the previous section, with corrections for the lepton energy and angular resolutions, the event selection efficiency, and the detector acceptance,  as described above, for each value of the centre-of-mass energy. The variation of these uncertainties with $\sqrt{s}$ is shown in Fig.~\ref{fig:variation}. 