I would just reset it at the trip pc. probably never been reset, I know people say disconnect battery, but when road service come out to a flat battery change ( I never once had a battery go flat on my i30 crdi's ) but when road service have to replace a dead battery they always plug in a small battery to keep the computers powered while the replace a battery. so could be risky doing that battery disconnect . like you might never have an issue, but always a chance.
the thing with the i30 or any other car's trip computer is, like all computers it does not have a brain!! if you are driving in heavy traffic and not moving and motor idles a lot the trip computer is useless as it uses distance to measure fuel economy. where time would be more accurate . very hard to explain. but say you go for a trip in country and non stop driving at a steady speed, after a while your trip computer could start to show you get 60mpg so adjust it's distance to empty to match. then you get to a big city the trip computer will still be telling you 60mpg but instantly you hit the city could be 29mpg but the trip computer does not know this. the only way to get closer to accurate in those conditions , would be to push the reset button it will start on present usage . but they are never 100% spot on but still a good thing to have as a guide.
just brim your new baby drive for a week or two depends the distance you normally cover then brim her again , when you brim reset your trip so you can work out your average on paper or what ever.
I ride motor bikes too and don't have a fuel gauge at all, I road my mb 3,800 km after buying it and was trying to work out the distance I could safely go on one tank as was over 300km ( about 190 mile ) in one spot that I could refill, by time I got home after the 3,800km ride I knew how far I could go on a primed tank so just used the trip gauge as no fuel gauge at all.