A flat tax isn't actually "flat" so long as there's a tax floor from which it starts. Say the first $30,000 of your income is free, the rest being taxed at idk, 25%. If you make $30,000 you pay no tax. If you make $50,000, you pay $5,000 in taxes, or 10%. If you make $100,000 you will pay $17,500 in taxes, or 17.5%. And progressively higher until you reach an actual tax rate of near 25%. This gets rid of any incentive to work less or get deductions, because there is always that same tax rate additional income will be taxed at.