If a person sells part of any patent rights in a chargeable period, and for that period U is greater than N, the writing-down allowance for that period is—

$\frac{U-N}{Y}$

where—

U is the unrelieved qualifying expenditure for the chargeable period,

N is the net proceeds of any sales of the patent rights which take place in the chargeable period (so far as those proceeds consist of capital sums), and

Y is the number of complete years of the writing-down period remaining at the beginning of the chargeable period.

If an amount is calculated under sub-paragraph (1) for a chargeable period, that amount is also the amount of the writing-down allowance for subsequent chargeable periods until another sale in a period for which U is greater than N causes a fresh calculation to be made under sub-paragraph (1).

If a chargeable period is more or less than a year, an allowance calculated under sub-paragraph (1) or (2) is proportionately increased or reduced.