The amount of a progress payment to which a person is entitled in respect of a
construction contract is to be—
(a) the
amount calculated in accordance with the terms of the contract; or
(b) if
the contract makes no express provision with respect to the matter—the
amount calculated on the basis of the value of construction work carried out
or undertaken to be carried out by the person (or of related
goods and services supplied or undertaken to be supplied by the person) under
the contract.