# 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 $$R_V = \frac{A_V}{E(B-V)}.$$ However I don't know how these terms relate to each other to apply them to my model spectrum (flux density).

1. 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?

2. 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?

3. 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 $$R_V$$.