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The first striking observation is that centre-of-mass energy is not particularly useful to improve the precision on the top-quark electroweak couplings. For four out of five of these couplings, the optimum is reach for $\sqrt{s} \simeq 360$\,GeV. The precision degrades by up to a factor four with 500\,${\rm fb}^{-1}$ at $\sqrt{s} = 500$\,GeV. It can also be noted that a very decent precision is alredy reached for $\sqrt{s} = 350$\,GeV. The second observation is that the precision reached for these four couplings is at the per-mil level, and that the tt$\gamma$ and the ttZ couplings can be determined independently with this precision without the need of initial polarization. (It was checked that, with this method, incoming beam polarization would actually significantly degrade the statistical power of the coupling determination, by up to one order of magnitude.)  It is only for $F_{1A}^Z$ that an increase of the centre-of-mass energy could improve the precision by a factor of two, from 2\% at $\sqrt{s} = 360$\,GeV to 1\% at $\sqrt{s} = 420$\,GeV. 420$\,GeV, an energy at which the single-top production would need to be included as a background to the study.  There are, however, many other observables to be studied in a ${\rm t\bar t}$ event. It was noticed that a factor of two improvement could be obtained at $\sqrt{s} = 360$\,GeV by including the energy and angular distributions of the b quarks, but this remark is beyond the scope of the present study.