Estimated velocity shift, Δv, computed on ten equispaced sections of the Lyman-α forest. Top panel: Δv estimated on each spectral chunk with the pixel-by-pixel (blue squares) and model-based (red diamonds) methods, as a function of the section's central wavelength λ_c. Middle panel: Measurement uncertainty, σ_v, for the two methods, shown with the same colour coding as the panel above. The dash-dotted and dashed grey lines report the expected uncertainty of the two methods, based on the median S/N and pixel number of each chunk. Bottom panel: Fraction of the Lyman-α forest $f_{{\mathrm{Ly}}_{\alpha }}$ considered in the measurement within each chunk (solid black line, left-hand axis), taking into account the masking of metals, strong absorbers and bad pixels. Note that the spectral chunk centred at 560.7 nm contains the sub-DLA absorber and has a significantly smaller fraction of usable pixels than the other regions. The right-hand axis of the same panel reports the median S/N at continuum level in each chunk in the first and second epoch spectra, shown with dashed green and orange lines, respectively.