You have a choice of which rate to use, so use the one that is most beneficial to you. According to the IRS:
Use the exchange rate prevailing when you receive, pay, or accrue the
item. If there is more than one exchange rate, use the one that most
properly reflects your income. You can generally get exchange rates from
banks and U.S. Embassies.
The IRS also lists the following resources for obtaining an exchange rate:
Governmental Resources
External Resources
