Schematic representation of the fitting procedure. For any U, N_H grid of single-cloud models, we computed the weighted combination of clouds in the grid that gives the best match with the observed lines. Usually, only a small fraction of the clouds in the grid have non-zero weights. The goodness of the fit was then quantified with a value of the loss function ℒ. The procedure was repeated for all U, N_H grids computed with different gas metallicities, and the behavior of ℒ as a function of metallicity was obtained. The metallicity of the observed spectrum is identified by the minimum value of ℒ. This procedure can be repeated with different ionizing continua, thus estimating the effect of varying the ionizing continuum on the metallicity estimate.