Back OT, it's impossible to determine what rate UA uses. Logically, it could be determined on the date of purchase, the date the money hits the account, or the date of the flight. Who knows? And the reason why you cannot determine it as you a) you don't know what sort of rate UA will use - interbank, tourist or anything imbetween, and you don't know how they breakdown the split between segments. I've tried to figure all this out (most of by tickets are purchased in GBP) but there are so many variables that it has not been possible to calculate.