Accuracy of recovered distance to neutral IGM, D_b, and local column density, N_H I as a function of S/N per pixel in mock NIRSpec prism and G140M observations. We show the median and 68% range of the maximum likelihood recovered values from 100 realisations. The low resolution of the prism limits the accuracy of constraints, while robust constraints could be obtained even for low S/N (∼5 per pixel) with G140M.