Positions of the n = 1 peaks observed at 345 GHz for all the possible emission configurations in the case of a black hole of mass M = 6.5 × 10⁹ M_⊙ described by a Schwarzschild metric (dark green), a RZ metric with a₁|_ϵ = 0 = 0.1 or 1 (dotted pink or starred pink), and a RZ metric with ϵ |_a1 = 0 = 0.1 or 0.5 (dotted violet or starred violet) and a Schwarzschild black hole of mass M = 5.8 × 10⁹ M_⊙ (light green). The velocity of the disc is taken to be mixed and the synchrotron pitch angle is computed in the emissivity. The red colour bar shows the error bar from the mean black hole mass value, M = 6.5 × 10⁹ M_⊙, while the grey colour bars indicate the value of the RZ parameters a₁ (pink) and ϵ (violet).