You can use this method for allocation of passive income (interest) that was received evenly through the year. However, when it comes to dividend and capital gain income. you have actual dates when these transactions took place. Therefore you should source capital gains and dividend income to the state for which you were a part-time resident when these transactions took place.
For example, if you moved from VA to DC on Dec 5th and had a capital gain transaction on June 4th and a dividend on Dec 31, then the capital gain would be considered VA sourced and the dividend would be considered DC sourced.
