(2) In accordance with Article 12, the competent authority of any Party shall determine:
(a)the amount of the theoretical pension which would be payable if all the relevant insurance periods completed under the legislation of all Parties had been completed under its own legislation;
(b)the proportion of such theoretical pension which bears the same relation to the whole as the total of the insurance periods completed under the legislation of that Party bears to the total of all the relevant insurance periods completed under the legislation of all Parties.
The proportionate amount thus calculated shall be the rate of pension actually payable by the competent authority.