Proper absorber separation from a wavelength interval ($\ell \simeq c\Delta \lambda /[H(z)(1+z){\lambda }_{\mathrm{rest}}]$)

A local redshift spacing between absorbers corresponds to proper path length $c\Delta z/[H(z)(1+z)]$. Using the same rest-frame transition for all lines gives $\Delta z=\Delta \lambda /{\lambda }_{\mathrm{rest}}$. This conversion assumes cosmological redshift dominates the line spacing and uses a local Hubble-rate approximation.