28. Where the Commissioners are satisfied that a person is not able to account for the exact amount of output tax chargeable in any period, he may estimate a part of his output tax for that period, provided that any such estimated amount shall be adjusted and exactly accounted for as VAT chargeable in the next prescribed accounting period or, if the exact amount is still not known and the Commissioners are satisfied that it could not with due diligence be ascertained, in the next but one prescribed accounting period.