[gamblor956]

For most currencies, you can basically use a single exchange rate for all transactions.

No, in determining f/x gain loss, you must use the exchange rate at the time of each transaction, though you get to choose FIFO or LIFO for determining that the base exchange rate is for determining gain or loss.

Generally, most businesses deal with f/x issues in one of two ways: either they have a foreign office that uses a foreign currency as its functional currency (a QBU) and process the foreign currency transaction through that office; or they have a bank handle the USD-to-foreign conversion on the day of the transaction at whatever the appropriate rate, is so there is no f/x gain/loss.