No you would calculate foreign workdays on all your RSU income. If 240 workdays during the year and 120 of the workdays were foreign you would calculate 120/240 * RSU income = foreign income from RSU. If you were working overseas in 2017, then your base salary can be allocated by workdays as well (but only on 2017workdays) Bonus would usually be allocated using prior year workdays if some of it was earned overseas.
