http://www.legislation.gov.uk/id/ukpga/2010/4

Part 3Companies with small profits

Marginal relief

22The ring fence amount

(1)

In section 21 “the ring fence amount” means the amount given by the formula—

(U R - A R) \times \frac{N R}{A R}

(2)

For the purposes of this section—

UR is the amount given by multiplying the upper limit by—
$\frac{A R}{A}$
AR is the total amount of any ring fence profits that form part of the augmented profits of the accounting period,
NR is the total amount of any ring fence profits that form part of the taxable total profits of the accounting period, and
A is the amount of the augmented profits of the accounting period.