5. The amount of tax to be repaid to a person referred to in paragraph 3 of this Order shall be so much of the amount of tax specified in paragraph 4 (d) of this Order as the person shows to the satisfaction of the Revenue Commissioners to be in excess of the amount which would have been borne or paid by him if the rate and percentage for the time being referred to in section 11 (2) (b) of the Act had applied to the supply or importation in question.