Answer to flyingsheep - be sensible
Yes, be sensible. BUT, keep one eye on the AFTLS as well because they do in fact apply to commuters as well, in that you are required to be fully rested for your duty, and the main reference that the courts will use (if something goes wrong) is CAD371.
If you land in HKG from Europe two hours before you sign on, I think it would be tough to argue you were rested in keeping with the intent of the regs.
I reckon 12 hours should be enough to be justifiable.