Convention here is to specify the extinction through the whole region, i.e. front to back.
A spline was fit to the data in Table 3 of RL85, and then interpolated. The law includes the 10 micron silicate feature. A spline was fit to the original data in Table 1 of M90, and then interpolated. The grain model consists of a mixture of graphite and 'astronomical silicate' grains, which obey an MRN size distribution. DL84 used Mie scattering theory for homogeneous spherical particles to compute the absorption and scattering cross sections.