No FLAT TAX. A FLAT RATE TAX is what you should be calling for (a flat tax would be a specific dollar amount for each individual to pay).
Simple tax proposal: Do away with the corporate income tax. Personal income tax will be a flat rate tax, no exemptions and no deductions, starting with the first dollar of income.
The rate shall be set by Congress and once set can not be changed, up or down, without a supermajority in both Houses of Congress. The new rate can not go into effect for at least two years from enactment.
As the current exemptions and deductions are a form of welfare, let Congress replace and fund a replacement system separate from the tax system.