I want to know the answer to the following question:
Under what condition is the approximation C0 sufficiently accurate for calculation purposes?
All the background I'm provided with is that "(Measurement "without the dielectric" in principle means measurement in free space, an unattainable goal inasmuch as even the quantum vacuum is predicted to exhibit nonideal behavior, such as dichroism.". Is that enough to answer the question?
Answer: no