Ratio of the approximate total surface density to the true surface density as a function of radius. Each line represents the ratio of an approximate total surface density to the true total surface density within 3h_z of the mid-plane for each galaxy as a function of radius. The black lines account for the radial derivatives in the Poisson equation (Eq. (8)) and the red lines neglect them (Eq. (7)). At radii where the bulge is significant, the red lines turn blue.