the compensation shall consist of an annual sum which shall be payable at intervals equivalent to those at which the emoluments of the claimant's employment are or were previously paid or at such other intervals as may be agreed between the claimant and the compensating authority, and shall, subject to the provisions of these Regulations, be payable until normal retiring age or death, whichever first occurs; and

the said annual sum shall not exceed the figure which bears to the maximum annual sum which could have been awarded under Regulation 15 of these Regulations had the claim been made under that Regulation, the same ratio as the amount by which his net emoluments have been diminished (calculated as an annual amount) bears to the amount of his net emoluments, so however, that no compensation shall be payable if this ratio is less than 2½ per cent.