Earlier quoted context omitted.
Because I like a challenge: =SUMPRODUCT( (bracket_min bracket_max)*(bracket_max-bracket_min)) ) *bracket_rate ) Where income is your income, bracket_min is the range of bracket minimums, bracket_max is the range of bracket maximums, and bracket_rate is the range of bracket tax rates. Demo on Google Sheets: https://docs.google.com/spreadsheets/d/1z0vx8TJeWr-hbJ3q6E7r...
Really cool! Unfortunately Numbers doesn't support more complicated SUMPRODUCT formulas...
=(income-INDEX(bracket_min,match(income,bracket_min,1)))*
INDEX(bracket_tax,match(income,bracket_min,1))+
INDEX(bracket_base_tax,match(income,bracket_min,1))