Andrew:

  document.getElementById("TotalIncome").value;

  var WeeklyIncome = TotalIncome / 52;
  var TaxableWeeklyIncome = WeeklyIncome - 183;
  var WeeklyNI = TaxableWeeklyIncome * 0.12;
  if (TotalIncome < 8632) WeeklyNI = 0;

  var NetIncome = 12500 + (TotalIncome - 12500) * 0.8 - WeeklyNI * 52;

  var RoundedNetIncome = NetIncome.toFixed(2);


Me:

  document.getElementById("TotalIncome").value;

  var TaxableIncome = TotalIncome > 12500 ? TotalIncome - 12500 : 0;
  var NIIncome = TotalIncome > 9500 ? TotalIncome - 9500 : 0;
  var NetIncome = TotalIncome - (TaxableI come * 0.2) - (NIIncome * 0.12);

  var RoundedNetIncome = NetIncome.toFixed(2);