Results of the simulations used to choose between different procedures to estimate the 2500 Å flux densities on 30 luminous quasars at z ≃ 2. We compared the flux density observed values with those obtained with each of the three methods described in the appendix. From the top left panel: rest-frame 2500 Å flux estimated assuming redshift 4.2, 4.4, 4.6, 4.8, 5.0, 5.2 (y-axis) vs. that estimated from the full spectrum (x-axis). The last two panels represent the comparison between the true rest-frame 2500 Å flux from the spectra, and that estimated from an extrapolation with a fixed power law, having the slope equal to 0.5 (Vanden Berk et al. 2001) and 0.79 (Vignali et al. 2001), respectively. The normalisation is assumed to be in the 1430−1470 Å spectral range.