19 CFR §159.33

If a rate of exchange has been proclaimed by the Secretary of the Treasury in accordance with 31 U.S.C. 5151(b) for the currency involved, such proclaimed rate shall be used unless it varies by 5 percent or more from the certified daily rate for the date of exportation as set forth in § 159.35. In determining the percentage of variation between the proclaimed rate and the certified rate, the difference between the two rates shall be divided by the certified rate.