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We use 62 185 quasars from the Sloan Digital Sky Survey (SDSS) DR5 sample to explore the quasar mass–luminosity plane view of virial mass estimation. Previous work shows deviations of ∼0.4 dex between virial and reverberation masses. The decline in quasar number density for the highest Eddington ratio quasars at each redshift provides an upper bound of between 0.13 and 0.29 dex for virial mass estimate statistical uncertainties. Across different redshift bins, the maximum possible Mg  ii  mass uncertainties average 0.15 dex, while Hβ uncertainties average 0.21 dex and C  iv  uncertainties average 0.27 dex. Any physical spread near the high‐Eddington‐ratio boundary will produce a more restrictive bound. A comparison of the sub‐Eddington boundary slope using Hβ and Mg  ii  masses finds better agreement with uncorrected Mg  ii  masses than with recently proposed corrections. The best agreement for these bright objects is produced by a multiplicative correction by a factor of 1.19, smaller than the factor of 1.8 previously reported as producing the best agreement for the entire SDSS sample.

The quasar mass–luminosity plane – III. Smaller errors on virial mass estimates