fisc-es.js

const steps = [
    {start: 60000, percent: .45},
    {start: 34000, percent: .37},
    {start: 20200, percent: .3},
    {start: 12450, percent: .24},
    {start: 0, percent: .19}
];

function calc(x) {
    for (let step of steps) {
        if (step.start < x) {
           return (x - step.start) * (1 - step.percent) + calc(step.start);   
        }
    }
    
    return x;
}

function getPercent(x) {
    return 100 * (1 - calc(x) / x); 
}