The determination of whether a judicial district or former judicial district has a population above or below 40,000 shall be made on the latest occurring of the following bases:
(a) As shown by the last preceding federal census of the district or of the aggregate cities and other political subdivisions situated within the district, whichever is greater.
(b) As shown by a subsequent census taken pursuant to Section 26203.
(c) As may have been found to be the fact in any proceeding for declaratory relief brought in a court having jurisdiction.