The amount of unified pension payable to an employee, former employee or survivor shall be the sum of:
(1) The amount computed according to this chapter, exclusive of service credited under § 5551(4)b. of this title; plus
(2) The sum of the amounts computed, based on credited service as an employee, according to subchapter II of Chapter 55 of this title; subchapter III of Chapter 83 of Title 11; and Chapter 88 of Title 11.