Non-monotonic behaviour of R_FeII versus log Un_H colour-coded with respect to log U (left panels). Corresponding cases for R_CaT are shown in the right panels. The panels represent the three sets of high-metallicity cases: log Z[Z_⊙]: 0 (top), 0.5 (middle), and 1 (bottom). The pale-blue vertical strip identifies the Un_H product yielding the reverberation-based R_BLR (black dashed line) within the uncertainties. The pale-orange horizontal strip identifies the observed R_FeII and R_CaT values (black dashed lines) from Persson (1988) and Marinello et al. (2016) within uncertainties. A column density of N_H = 10²⁴ cm⁻² is assumed.