If you are trying to calculate the amount of the RSUs that are foreign source income then you have to be able to decide how much of them were issued from a foreign source.
It sounds simple but the answer to your question really isn't. Your RSUs were issued to you in Taiwan from an American company. If that American company issued you a US W2 then that makes them US source income. And the only deduction you receive is for taxes that you paid to Taiwan. And if the RSUs are all US source income then your foreign tax carryover applies but only to the US source for which it was taxed.
In your position - as complex as your situation is - I would recommend sitting down with a tax professional and letting them take a look at all of your paperwork. Your problem may really be beyond the scope of this forum.
@chenm003
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