I would use the manual method. It certainly is the most accurate over time and will give you a better feel for how you're doing. I tried to back calculate using the indicated mpg versus my actual and over 6800 miles the gage is about 3 .5 gallons off. That's not a lot of gas but it's enough to have put my average about 0.4 mpg higher than what I'm calculating.

I have tanks that are right on the money and some that are 2 mpg off. Over the long term the manual will give you better numbers. The screen average, assuming someone does not push the all clear button will be a little off. If someone pushes the all clear you're out of the game.

Due to the error I wouldn't mix and match the screen measurement with manual entries as you may drastically under / over state the actual overall.