you press the reset button under the dials on the dash. Once resets some of the data, twice resets all of it.
I reset all at every fill up. I get about 32 in normal driving (mostly short runs), up to 36+ on motorways. I also find that if you do a hand calc, the trip computer is usually wrong. Typically the real mpg is about 1-2 mpg less than the computer would have you think.
I usually get around 300 miles to a tank, maybe another 40 with motorway driving. I'm never really sure how much is left though TBH. My fuel gauge doesn't move for 40 miles, then goes down a quarter in about the next 500 yards, then sticks for a bit and then drops another quarter. It's all over the place!