How does Rav Moshe Feinstein define the calculation of Ma'aser?

Rav Moshe Feinstein's approach to the calculation of Ma'aser emphasizes the idea that since taxes are not considered property owned by the individual, one should calculate Ma'aser based on income after tax obligations have been fulfilled. This reflects a broader principle in Jewish law that focuses on what an individual actually retains or benefits from after obligations to the government are met.

Calculating Ma'aser in this manner allows individuals to give tithes based on their actual financial capacity to contribute, promoting a fair and honest practice of charitable giving. This perspective also aligns with the understanding that Halacha recognizes the reality of societal obligations like taxes, thus separating them from personal income that is available for mitzvot.

The other options do not align with this interpretation. Calculations based on pre-tax earnings would not accurately represent what is genuinely disposable income, while calculating Ma'aser solely on business revenues could potentially disregard personal income considerations and other necessary expenses that affect actual earnings.