Predicted ratio of broad Hβ to HeII λ4686 emission as function of ionization parameter log U, based on the sub-Eddington CLOUDY models listed in Table 1. Blue and orange curves correspond to the Jin et al. (2012) and Pezzulli et al. (2017) SEDs, respectively. As the ratio is independent of the covering factor, only one curve is shown per SED. The upward-pointing green arrow marks the 3σ lower limit on the broad-line Hβ/HeII ratio measured in the z > 5 grating stack.