While reporting Tax Exemption, how to determine the foreign currency exchange rates

Production for export goods enterprise declares "Exemption, Set-off and Refund".  How to determine the foreign exchange rates?

According to the "PRC VAT Tentative Regulations" Article 15, taxpayers’ currencies other than RMB clearing sales, its sales of RMB conversion rate can be selected at the time of sales occur on the 1st day of the month or the RMB the central parity rate. Taxpayers shall determine in advance the conversion rate used to determine, but not to change after one year.

Therefore, it is the export enterprises to declare the export refund (exemption) should be required to determine the foreign exchange rate.