Comparison between CLOUDY synthetic transmission spectra of the Hα line before (top) and after (bottom) normalisation, computed considering different reference pressure levels at 0.001 bar (black), 0.01 bar (red), and 0.1 bar (blue). For reference, in the bottom panel the green line shows the observed Hα transmission spectrum, while the straight black line shows the average 1σ uncertainty obtained from the observations. Wavelengths are in vacuum.