10. In the case of any payment in respect of which income tax is chargeable under section 123, relief shall be allowed by means of deduction from the tax chargeable by virtue of that section of an amount equal to the amount determined by the formula -
where -
A is the amount of income tax which apart from this paragraph would be chargeable in respect of the total income of the holder or past holder of the office or employment for the year of assessment of which the payment is treated as income after deducting from that amount of tax the amount of tax which would be so chargeable if the payment had not been made,
P is the amount of that payment after deducting any relief applicable to that payment under the preceding provisions of this Schedule,
T is the aggregate of the amounts of income tax payable in respect of the total income of the holder or past holder of the office or employment for the 3 years of assessment preceding the year of assessment of which the payment is
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