I don’t either (mines a diesel and that can be a problem if the tank runs dry). But I did, deliberately, drive locally as long as I dare to see where ‘empty’ was. Even with the last ‘bar’ split to only show the last little bit of gauge, I could only get 33 litres in.
I usually drive with the dash display showing ’range’ and fill up when that gets to the point where ‘35 miles’ turns to ‘- - -‘
On a run I make regularly to Norfolk, I can set off with it showing range of 380 miles. During the 100 mile trip that can go up to showing 450 miles, and often I arrive with more range than I started with! All a bit vague.