Midplane solid-to-gas ratio ϵ shown as a function of α_turb for various values of Z, according to our very simple model that is applicable when particles are well bound to the gas. The vertical and horizontal dashed lines show our empirically found critical α and ϵ values, respectively. We hypothesize that combinations of α_turb and Z that lie below the horizontal dashed line will produce insufficient particle densities at the midplane to form planetesimals via the SI/SG mechanism. We emphasize that this figure is generated based on a single, empirically determined value from our study in combination with Equation (34) and should be considered a toy model that extends our results under some naive assumptions.