primary selection criteria were, firstly, to restrict the B supergiant sample to stars with $v_\infty$ $\simlt$ 970 km s$^{-1}$ so that the two components of the Si[iv]{} $\lambda\lambda$1393.76, 1402.77 doublet can be treated as radiatively decoupled, since the radiation from one component cannot interact with the radiation field of the other (see e.g. Prinja [&]{} Massa 2010). This sample of high-resolution $IUE$ spectra was then further restricted to stars that exhibit clearly defined, well-developed NACs close to $v_\infty$. We rejected cases of very deep NACs which would not allow the radial optical depth ($\tau_{\rm{rad}}(w)$) to be reliably constrained. We also rejected stars with very narrow (full width at half maximum less than 0.05 $v_\infty$) NACs since the velocity width of these features is less than the resolution of the optical depth bins used in the line-synthesis; see below. Unfortunately only 6 B supergiants fit these criteria (see Table 1), so in order to increase the sample size we relaxed the criteria slightly to allow additional cases where the $v_\infty$ is $\simlt$ 0.65 of the Si[iv]{} doublet separation. Thus an additional 6 stars with good NAC examples were added (Table 1). Though strictly the doublets in these additional stars can interact, 