We present a method for obtaining accurate black hole (BH) mass estimates from the MgII emission line in active galactic nuclei (AGNs). Employing the large database of AGN measurements from the Sloan Digital Sky Survey (SDSS) presented by Shen et al., we find that AGNs in the redshift range 0.3-0.9, for which a given object can have both H-beta and MgII line widths measured, display a modest but correctable discrepancy in MgII-based masses that correlates with the Eddington ratio. We use the SDSS database to estimate the probability distribution of the true (i.e., H-beta-based) mass given a measured MgII line width. These probability distributions are then applied to the SDSS measurements from Shen et al. across the entire MgII-accessible redshift range (0.3-2.2). We find that accounting for this residual correlation generally increases the dispersion of Eddington ratios by a small factor (~0.09 dex for the redshift and luminosity bins we consider). We continue to find that the intrinsic distribution of Eddington ratios for luminous AGNs is extremely narrow, 0.3-0.4 dex, as demonstrated by Kollmeier et al. Using the method we describe, MgII emission lines can be used with confidence to obtain BH mass estimates. ; Comment: 10 pages including 4 figures, submitted to ApJL, incorporates referee's comments