The assessment required in any year to meet the payment due to the United States for all purposes under the contract may be apportioned in accordance with the benefits. In the ascertainment of the benefits there shall be taken into account:
(a)
The provisions of the contract, the applicable Federal laws, and the notices and regulations issued in pursuance of those laws.
(b)
In addition, in case the contract is for the assumption by the district as principal or guarantor of indebtedness to the United States theretofore existing on account of district land, the provisions of existing contracts carrying the indebtedness and the amounts of the liens that may be released in pursuance of the contract.