Tax relief for past income under book profit adjustments follows a prescribed formula and Form No. 53 filing requirement. Relief in tax payable under section 206(1) is computed by reducing the tax payable under a prescribed formula comparing book profit with and without past income, and comparing past-year tax after and before giving effect to that income. Past income is limited to income included because of an advance pricing agreement or a secondary adjustment. Any negative result is treated as zero, the corresponding tax credit under section 206(1)(m) is reduced, and the claim must be made in Form No. 53 with verification by the authorised person.