There is no official mid-market rate, and XE.com is definitely not a source of an interbank rate. The most reliable free source of live wholesale interbank rates is at
http://www.truefx.com which closely resemble the rates on EBS, Reuters Spot Matching, Currenex etc. The card networks could do their trades at any time of the day; you don't know when that might be and what the rate might be at that moment.
The rest of your logic is correct.
Fair enough, I'm no expert on the topic! I was just meaning that their historical data says it's the mid-market rate so I assumed it was reasonably accurate - I wasn't looking for live data as I needed to pick a previous date to compare across the various companies. I only picked a single day and things may be different on different days, but it hopefully illustrated the point that Amex don't have a great rate!
In day-to-day use I tend to just guesstimate the amount I'll be charged based on XE and normally find with my PO MC it ends up being less ^
