Blue solid lines in both panels show the histograms of the difference between J-PLUS r flux (i.e. ) and obtained from SDSS QSOs spectra (see Fig. C.1). The distributions are normalised by respectively (left panel) and its photometric error (right panel). Both distributions are centred at zero (see plot legends), meaning that and values are statistically equivalent. On the other hand, the distribution spread in the right panel is significantly bigger than one, and therefore photometric errors do not fully account for the flux difference. Consequently, we re-scale to the value obtained by the Gaussian fit (right-panel legend).