4. Threshold at which additional rate of income tax is charged

That provision may be made reducing the amount of the higher rate limit in section 10 of the Income Tax Act 2007 so that it is equal to—

(a) twice the amount specified in subsection (1) of section 35 of that Act (personal allowance) for a tax year, plus

(b) the amount specified in subsection (2) of that section (amount at which personal allowance starts to be withdrawn).