I hadn't noticed that the ratio tracker wasn't there with the Outback. It was with my past cars.
Under the edit page for the vehicle, there is a toggle to track city/highway or not. If set to yes, a slider bar will appear on the fill up page.
EPA city is stop and go traffic; average speed is 21mph. EPA highway is not what people think of as highway today; most would likely call it country roads. Which leaves what to enter here as guess work.
A consistent way is what one poster here used. Their average speed for the tank, which they compared to the average speeds of the tests. Won't help if you don't have a way to track that average speed.
EPA test details are here,
https://www.fueleconomy.gov/feg/fe_test_schedules.shtml
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