Distribution of Galactic extinction A_X (in magnitude) for p_B5D = 0.1 for a library of 2604 extragalactic source SEDs and a set of optical-to-near infrared filters. Top panel: same as Fig. 8 for three values of R_V (=2.9, 3.1 and 3.3). The inset panel shows the F99 extinction curves for R_V = 2.9 (dotted), 3.1 (solid) and 3.3 (dashed) along with the filter throughputs. Bottom panel: median of 100 realisations of the distribution of Galactic extinction A_X for a library of 2604 extragalactic source SEDs to which we apply a pseudo-random value of R_V from a normal Gaussian distribution with a mean of R_V = 3.1 and a standard deviation of 0.2 in order to simulate random Milky Way lines of sight dust properties (at fixed p_B5D value).