shall be computed on the basis of the operating budget of the housing authority as approved by the department with provisions for a full operating reserve. Said rentals together with all other available moneys, revenues, income and receipts of the authority, from whatever sources derived, and together with the requisite annual contribution, will be sufficient (a) to pay, as the same become due, the principal and interest on the bonds of the authority; (b) to meet the cost of insurance and the payments in lieu of taxes provided by section sixteen and to provide for maintaining, operating and using the projects and the administrative expenses of the authority; (c) to create, during not less than the twelve years immediately succeeding its issuance of any bonds, notes or other evidences of indebtedness, a reserve sufficient to meet the largest principal and interest payments which will be due on such bonds in any one year thereafter and to maintain such reserve; and (d) to provide such tenant services for residents of housing projects as the department may approve.