The online filing software automatically adjusted my overseas salary income (category 5F) for the days I’ve physically been in Taiwan.
I.e. if you make 1 million, but only stay 300/365 days, you only need to pay tax on 822k.
They will also accordingly adjust the exemption accordingly.
I would assume it works like that for non-salary income accordingly. You might even be able to provide proof about specific income items - bit I’d assume that the simple distribution by days should be more beneficial in case you also enjoy some vacation days overseas.