Upon the liquidation of a domestic reciprocal insurer, its assets remaining after discharge of its indebtedness and policy obligations, the return of any contributions of the attorney or other persons to its surplus made as provided in section 20-778, and the return of any unused premium, savings or credits then standing on subscribers’ accounts, shall be distributed to its subscribers who were such within the twelve months prior to the last termination of its certificate of authority, according to such reasonable formula as the director may approve.