From my experience, it's not consistent even during a posting day.

I ran two transactions on two different cards of identical amount within seconds of other with same merchant <-- same rate

I then took one of the cards for lunch a little later <-- slightly different rate, even though same posting date

So I think the rates posted in https://www.mastercard.com/global/cu...ion/index.html and http://www.visaeurope.com/en/cardhol...nge_rates.aspx
are indicative not conclusive. You can't get the exact rate stipulated on those pages, but from my experience and statement analysis, you won't go far from them either (you'll be within 0.10% of them, worse if your transaction is larger in my experience).

We in HK also kept a running thread of what our banks charged us for foreign currency, and the spread over the HK Association of Banks' rate that day http://www.hongkongcard.com/forum/fo...ow.php?id=3939