Thanks Ekamol,
On the 400,000 gross you are paying only 5% more in income tax at years end.
What is the withholding tax? Is it income tax but called withholding tax?
If PSI withhold each month, you will not have to pay any income tax as it is done throughout the year. Any adjustment may be done by PSI at years end to make up balance.
I will review tax law for Thailand and advise.