For the purposes of this Part the “net taxable specific income” from an employment for a tax year is given by the formula—

$\mathrm{TSI}-\mathrm{DSI}$

where—

TSI means the amount of any taxable specific income from the employment for the tax year, and

DSI means the total amount of any deductions allowed from that income under provisions of the Tax Acts not included in the lists in section 327 (3) and (4) (deductions from earnings: general).

If the amount calculated under subsection (1) is negative, the net taxable specific income from the employment for the year is to be taken to be nil instead.

If a person has more than one kind of specific employment income from an employment for a tax year, the calculation under subsection (1) must be carried out in relation to each of those kinds of specific employment income; and in such a case the “net taxable specific income” from the employment for that year is the total of all the amounts so calculated.