Yes, thanks. I understand the calculation process. I just don’t understand the function logic.
For an self employer, he should deduct the NIC1 secondary (13.8%) which is deductive before income tax and NIC 4.
Therefore i am not quite sure the logic of function 13.8%*(20%+9%)=4% is right or not. since the logic of this function seems to indicate 13.8% is after income tax and NIC4