Comparison between continuum-subtracted FLITS spectra (black), ray traced for disc inclination 45°, and spectra directly obtained from our vertical escape probability method (red, “EscPro”), both convolved to R = 600 spectral resolution. The agreement is remarkable for the main model with gas/dust ratio g/d = 1000 and inner disc radius R_in = 0.07 au (upper panel), where disc surface emission dominates. However, for the truncated disc model with g/d = 100 and R_in = 3 au (lower panel), the escape probability method fails because the line photons are mostly emitted sideways through the inner disc wall, in which case the EscPro spectrum results to be too faint, and too red.