For civilian U.S. citizens outside of the U.S., the voting residence is the "last U.S. residence" or the last place you lived (were domiciled) before leaving the U.S. The voting residence does not depend on where you last voted.
Under traditional rules and state law, a person is permitted to register and vote only in the place that constitutes his/her domicile. Different rules apply to active duty servicemembers, but when a civilian moves from one place to another with the intent to make the new place home for a significant time, the individual's domicile changes to the new place. Leaving place A to go to place B for a temporary purpose, however, with the intent to return to place A or to move to still another place when the temporary purpose has been fulfilled, does not amount to a change of one's domicile.