I agree with MMM's way of calculating savings rate, because it provides the savings % that allows the saver to most accurately calculate years to FIRE, using the table in the same Simple Math article. Using MMM's way, as I understand it:
1. Yes, Social Security contributions are a tax. They should be excluded from the after tax income. For MMM purposes, the income consists of two components: discretionary income used for living expenses, and discretionary income used for investing. Social Security contribution is not discretionary, it is mandatory, so subtract it from income in order to prepare for the savings rate calculation.
2. Yes, mandatory insurances are a tax for this calculation. Reasoning similar to Social Security.
3. Yes, health insurance the same, because mandatory.