Simple, a rational tax system might only tax a person's discretionary income, much like a business is only taxed on its actual net income. In such a scenario, a taxpayer would not owe taxes on monies that they were required to pay to third parties, such as taxes, or required insurance premiums, or child support, etc.
The rate is independent of the question of what constitutes taxable income, conflating them is not beneficial to society, but helps politicians in their game.
Please define discretionary income, in numerical terms. The truth is, you can’t.