420 miles isn't that far to travel on a full tank if you are mainly not doing urban stop-start driving.
I drive a Fiesta with the same Ford 8 bar gauge. They are not particularly accurate - I live in a mountainous area and the reading can fluctuate between 5 and 3 depending on whether I'm going downhill or uphill.
Given that the dash indicator gauges aren't that great, the first thing any sensible mechanic will ask you to do is fill up to the maximum (the very maximum) and then drive until the low fuel warning light comes on, then fill up again and do the same, keeping a mileage log.
If you do that before you go to a garage you will earn their respect. (If it then seems that they don't know what you are talking about be very wary - but consider that you might just be talking to the 'work experience' person on their first day.
BTW when the Fiesta seemed to be drinking fuel and was due a service the local garage/Ford dealer installed the latest update of the engine software. It was definitely worth it.