Where:
(a) but for this section, a person would, in relation to a petroleum project, derive for the purposes of this Act an amount (in this section referred to as the assessable amount ) of assessable property receipts, assessable miscellaneous compensation receipts, assessable employee amenities receipts or assessable incidental production receipts in relation to property in respect of which eligible real expenditure was incurred by the person in relation to the project; and
(b) the Commissioner considers that, because section 42 applied in relation to the eligible real expenditure or for any other reason, a proportion only of the assessable amount is attributable to the eligible real expenditure;
the person shall be taken for the purposes of this Act to have derived only that proportion of the assessable amount.