(1) Where -
(a) the total income of any individual from all sources, whether chargeable with income tax by deduction or otherwise, includes income from any source or sources which is to be computed on the basis of the actual amounts receivable in the year of assessment or where any deductions allowable on account of any annual sums paid out of the property or profits of an individual are to be allowed as deductions in respect of the year in which they are payable, and
(b) an assessment to income tax is being made before the end of the year of assessment to which such assessment to tax relates,
the inspector in making the assessment shall, in computing the total amount of income assessable to income tax, estimate the amount of income from each such source or the amount of any such allowable deductions and, in making any such estimate, the inspector shall have due regard to any corresponding amount of income or allowable deductions in the year preceding the year of assessment and sh