Your statement "not applicable here" is a policy decision, not an absolute fact.  It may have been policy for a long time and many people may accept it as fact, but it is not.  My preference is to eliminate all income-based taxes for a new way to fund the federal government.  These new revenue sources are much more efficient, avoid high tax rates and even filing of returns, but I won't go into them here.  The AGI includes deductions that are other policy decisions, so AGI isn't applicable here because I'm talking about a non-income tax system with very low rates and few if any deductions.  The $16 Trillion is the number to use, even though it includes social payments, because it mostly is the difference between wage and non-wage income.  SS is $902 billion, leaving nearly all of the $16 Trillion left.  Current policy even taxes SS income, so it is not off limits.  Taxing the $32.4 Trillion Gross Output and most personal income results in very low 3% rates to be one of four new taxes.