Theoretical values of the ionization parameter U and the computed column density N_H from N_ion as derived from Eq. (4) (see Sect. 3.3). Solid and dashed lines correspond to the values of N_H derived using C_f^min and C_f^max, respectively. The purple curves show the N_H constraint from CIV, while the green and orange curves represent the upper limits from NV and Lyα, respectively. Bold lines indicate the allowed solutions for N_H and U, while the red crosses mark the median values of the solutions adopted in Eq. (5).