The US way of shoe sizing is random at best. As you noticed, Mondo is an actual measured number.
I measure a 25 on one foot, and a 24.5 on the other. Make sure you check both feet.
Now, I tried a 25 at first and it was way too big. Next year I went to a 24, which was slightly better. This year I finally went to a 23, and it feels great. These were all Deeluxe boots. I found it a lot easier to fit into a smaller shell, than it was to make a larger shell work.
Get yourself to a good bootfitter. Like jp1 says, it's worth a drive.