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Injection of electromagnetic waves in the electron-cyclotron (EC) frequency range is one of the most promising schemes to drive part of the plasma current in a tokamak fusion reactor. The theoretical calculation of the driven current, as usually performed by ray/beam tracing codes, relies on the knowledge of the magnetic equilibrium, the electron density, and temperature profiles on the one hand and of the wave injection parameters on the other. If the optimum current drive efficiency for a given scenario is sought, extensive parameter scans are usually performed to determine the best injection conditions. This is, however, not a viable approach in typical systems-code applications, where the plasma configuration is not provided in sufficient detail and parameter scans would be anyway too demanding from the computational point of view. In this case, a different approach is required. In this paper, a procedure for the evaluation of the optimum current driven by EC waves for given global parameters is proposed, which relies on a single numerical calculation of the current drive efficiency, based on the adjoint method (including momentum-conserving corrections). The results are shown to be in good agreement with the full numerical optimization of the EC current drive efficiency for a variety of reactor relevant scenarios. This simplified approach also helps clarify the physics underlying the optimum current-drive conditions and the limitations to the achievable current-drive efficiency in reactor-grade plasmas. [ABSTRACT FROM AUTHOR]