Provisional tax and setting money aside
The “monthly set-aside” is not a tax bill instalment
A Hong Kong tax bill usually includes the final tax for the year and provisional tax for the following year. The tool’s monthly set-aside simply divides one year’s estimated tax by 12 to help with cash flow; it does not predict the first or second instalment on your tax bill.
If your income falls sharply, you stop working or another statutory ground applies, you can apply to the Inland Revenue Department, within the time limit, to hold over provisional tax.