How do I calculate exchange rates?

You can use either a spot rate based on the exchange rate on the day you received the funds (although if doing so you must use spot rates only throughout the return) or you can use the year end published dates found here.

When completing the questionnaire, it is fine for you to list the items in the foreign currency (just make sure to specify what currency they are in) and your Greenback accountant can do all the conversions for you.