Panel a: absolute values of the differences between the outflow velocity values determined with the different observed chromospheric profiles shown in Fig. 2 as a function of the Doppler factor. The abscissa axis is complemented with the velocity values corresponding to the Doppler factor in the case of the Auchère (2005) profile at the top of the panel. Panel b: same as in panel a, but with the parametrised chromospheric profiles shown in Fig. 2 and in panel a of Fig. 6. Panel c: same as in panel a, but taking into account the profiles shown in panels a and c of Fig. 7.