product of mass-loss rate and ion fraction, $q($Si$^{3+})$. Mean values determined over the NAC velocity region are listed in Table 3 for our sample stars. These estimates may be compared to $q($Si$^{3+})$ calculated at lower velocities ($\sim$ 0.4 to 0.6 $v_\infty$), and we also provide in Table 3 the mass-loss rates obtained from the model prescriptions of Vink et al. (2000, 2001), $\dot{M}({\rm Vink})$. Changes of $T_{\rm eff}$ or $L/L_\odot$ resulting from a $\pm$ one spectral or luminosity bin error, would change these mass-loss rate predictions by less than a factor of 2.

It is clear that $q($Si$^{3+})$ is higher when measured in the NACs; in the ’poster-child’ cases of HD 13866 and HD 47240 (which have excellent NAC profiles) $q($Si$^{3+})$ is larger by factors of $\sim$ 9 and 5, respectively. Further, recall that some stars in our initial sample were discarded because the NACs were saturated, so these values should be considered lower limits.

There is a second consequence of porosity that also affects derived mass loss rates. Whether NACs are due to an accumulation of clumps (Sundqvist et al. 2010, Šurlan et al. 2012) or CIR arms (see Fig. 12 in Fullerton et al. 1997) in velocity space, 