Effective prior probability distribution for the external convergence κ _ext from combining ray tracing through the Millennium Simulation with (1) the galaxy count around the lens system relative to the average number of galaxy counts, and/or (2) the modeled external shear of 0.089  ±  0.006. Dashed line: the convergence distribution for all lines of sight; dot-dashed line: the convergence distribution for lines of sight with relative galaxy count n _r = 1.4  ±  0.05; dotted line: the convergence distribution for all lines of sight weighted by the likelihood for γ _ext from the lens model; solid line: the γ _ext-weighted convergence distribution for lines of sight with n _r = 1.4  ±  0.05. The effective prior for κ _ext used in the final cosmological parameter inference is described by this most informative distribution.