Broad Hβ EWs as function of ionization parameter log U, derived from sub-Eddington CLOUDY models (parameters listed in Table 1). Different line styles indicate varying broad-line-region CFs, ranging from 1.0 to 0.1; models with the same SED share the same color. The shaded region for each SED represents the envelope spanned by CF values from 1.0 down to 0.1. The horizontal dark violet line marks the observed EW of broad Hβ measured in the z > 5 grating stack. The vertical axis uses a symlog scale (linear below 200 Å, logarithmic above).