The quantity you want is basically the extinction law, and is usually called . An extinction law is a fit to several measurements of the extinction in some direction (or an average of several directions).
Cardelli et al. (1989) provides different functional forms for the mean extinction law, parametrized in their Eq. 1 as
where is the inverse wavelength in , and the coefficients are given separately for IR, optical, UV, and FUV in Eqs. 2, 3, 4, and 5, respectively. The total-to-selective extinction takes different values for different lines of sight, but usually lies in the range 2.5 to 6, with 3.1 being a typical value in the Milky Way.
To get the quantity you're interested in, simply convert your favorite wavelength to , stick into Eq. 1, and multiply by :
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