When I arrived in HK I saved what I owed x2 as my actual payment owed to the IRD got doubled as I had to pay provisional taxes.
In subsequent years you can ask the IRD to lower the provisional tax if you expect a drop in income. I was going to work on a start up and this was accepted.
As a foreigner, on leaving employment, usually they withhold your usual salary cheques unless you are not leaving HK. Being a PR is one way, another, which I did, was to sign an undertaking to my employer that I would be managing my own tax affairs directly with the IRD.