Welcome to Prius Chat. Hope you found your answer from Alan helped. Personally I have never bothered with that facet of the display. I'm just happy that my fuel bills are half of what they were with my previous cars.
[“So tell me HOW can the Prius make ANY kind of estimate about another car?”] • Because you tell your Prius this info [“Furthermore, HOW can the Prius accurately know "how full" I filled the gas tank?”] • Seriously?
Just fully charge the Prime and a full tank of gas. Use the electricity as the reserve for the gas tank. The EV range is very accurate so use it last. Bob Wilson