Method used to process exchange rate with AMEX Cards

I made some foreign transactions and would like to know what AMEX considers the "base rate" plus the 2%. What is the base rate?
"Foreign Currency conversion rate is base rate plus 2%."

American Express uses the exchange rate of the day a charge is processed (plus a 2% fee assessed after the conversion rate is applied), not the rate of the day a charge is incurred.

Most Cardmember charges are converted to billing currency using Interbank rates as reported by the Reuters information system. Foreign exchange rates not available on Reuters are established by contacting local banks or by referring to published Interbank, official or tourist rates. All other foreign exchange rates are reported to us by local paying agents and are generally commercial bank rates, or the "Official Rate" where required by law. If the Reuters system is used, there is a cut off everyday at noon (Eastern Standard Time) for the daily rates.

The following link will allow you to make the conversion using the Interbank rate. However, it should only be used as a guide, as rates are subject to change.

Oanda.com provides a source for calculating conversion. Also, read more on international travel with an AMEX Card.