# Clarity on dust extinction terms and implementation: R_V, A_V, and E(B-V)

I have created a model galaxy spectrum using STARBURST99 and want to compare it to an observed spectrum of a particular galaxy. I understand that I need to redden my model spectrum to do this, so I am following the Cardelli et al. (1989) parameterization, which returns A(lambda)/A(V) for different wavelengths. I understand that the extinction correction in magnitudes is A_V, and I know that R_V = A_V/E(B-V). However I don't know how these terms relate to each other to apply them to my model spectrum (flux density).

I've read that the commonly adopted R_V value for the Milky Way is 3.1, but what is the E(B-V) value? If I want to apply extinction to a model galaxy for both the Milky Way AND the other galaxy's internal extinction, how would I go about this? For the external galaxy, I have a measured E(B-V) value to use (or does this only apply to the MW extinction?), but in this case I don't know what to use for R_V.