Refractive index is used to calculate phase velocity of light in medium, other than vacuum. Recently I had a discussion with somebody claiming that light is slower due to magnetic field of atoms.

I have carefully read the question and answers about the reasons why light appears slower in mediums. I have also been searching the interned on this matter before.

But my question is not why it slows, but why the refraction indexes have the values they have.

If you were unable to perform any measurement, but knew the atom properties (and material density which is apparently relevant), would you be able to calculate that index?