Calibration of MPG shown on 2010 Prius

I don't believe calibration as such is even possible. My 2009 has a fuel indicator that is often wrong and it's random. Calibrating a meter that misses the mark by a random amount would make no sense. I always chucked it up to the bladder that's in the tank but I don't know if that's what actually happens - my

2006 second gen Prius I had before had the same tank design and the fuel gauge was pretty accurate nevertheless.

I always go by the "miles since last fill-up" counter that's on the bottom of the MPG screen (mine is a previous generation Prius tho)

Yes it can, but it won't be easy, by adjusting the fuel pressure slightly. Displayed MPG is calculated based on miles driven, and fuel used. Since fuel used is measured by total fuel injector "open" time if you change the pressure the pulse width will be changed to compensate hence the displayed MPG will change.

Reduce pressure, the ECM increases the long-term fuel trim, hence the injector milliseconds, it thinks it injected more fuel, so the MPG number goes down.

Increase fuel pressure and the opposite happens.

Since the fuel pressure regulator is in the fuel tank this will be difficult.

This sounds like an epitome of don't-try-this-at-home projects. I don't know if the OP asked about something he/she could do him/herself but I would not mess with fuel injector only to see the proper fuel gauge reading. After all, you do have the second source of the same information - the MPG reading and the miles since fill-up. Divide miles by MPG and you get the gallons used. I've never been able to put more than 8.5 gallons in the tank and even then the gauge showed the tank just about empty.

I didn't say anything about the fuel level gauge, just the average MPG readout.

That first assumes consistent inaccuracies, not random. Second, what effect will changing the pressure have on power, emissions, and actual fuel economy? Even if you could change the pressure, the effects could be unacceptable.