I made it! (To the gas station, without running out.)
And I also learned I'd been operating on the incorrect assumption that the MPGiata had a 40 L tank... it's a 45 L tank.
So I "only" put 41 L in. Turns out there was nothing to be worried about after all!
That means I can theoretically do a kilotank (1000 km) based on my recent MPG. A project for next summer! I'll bring along a measured amount of fuel in a jerry can near the end. Frank will be proud of me.