Relative deviations, True − Fitted/True × 100%, of doubly exponential and change throughout for consistency disc models as a function of z₀ and z_e (z_e = 300 pc orange, 100 pc violet, and 20 pc pink), fitted by a z₀ = 0 model, assuming the correct R_e. The yellow region determines the 1σ uncertainty from the z₀ fit from 1.8 MeV data alone, see Sect. 3. By definition, the z₀ = 0 case yields a relative deviation of zero.