Mean values (top) of the logarithmic X conversion factor for all A_V-PDFs, and the corresponding standard deviation around the mean (bottom). Top panel: average log₁₀ $\overline{f_{\text{DARK}}}$ fraction for all ζ_CR − χ/χ₀ pairs at constant [O/H] and A_V-PDF properties. Bottom panel: effect of the ambient environmental parameters of the ISM in determining the f_DARK. The large drops of σ in the m = 10, 15 at σ = 0.2 (orange and green solid lines) are because for these distributions, a nearly uniform value of f_DARK is obtained for any pair of χ/χ₀ − ζ_CR.