Democracy 3 Income Tax Question

I want to adjust USA's income tax to reflect Sander's proposed income tax brackets, but there is only one slider for income tax. This leads me to my question: what does the income tax percentage actually represent? Is it the rate that the top bracket is taxed at, or is it some sort of aggregate of all the tax brackets. If it is the later, what math must one do to arrive at the initial income tax percentage?