Percent of income to go to premium
HR 3590 Section 1001 HR 4872
Code of Federal Regulations §1.36b-2Eligibility for premium tax credit.
Fair share means that even if your income is below 400% of Poverty Level, if you can get the Silver Plan for less than 9.5% of income, you don’t get a subsidy. Cahba.com The Kaiser Foundation Calculator does a pretty good job of showing more detail of how this is calculated.