Depending on driving style I have seen about a 10% error on Jazz mpg readout vs real life fuel used. Agressive driving delivers the worst error, steady driving the lowest error. (display only updates every 10 seconds and takes a 'snapshot' of the rate fuel is being used at that time - so if driving is erratic it will sometimes miss some of the peaks during rapid acceleration)
If you are going to work out the mpg yourself it is best to zero the trip meter every time you fill up, otherwise the previous mileage and fuel kinda muddies the equation. You may notice that after the 'trip' is reset the 'mpg' display reacts very quickly to (for example) going up and down hills, but as the trip mileage rises the rate of reaction slows down, this can only be because the computer is taking all previous fuel and mileage into its calcualtions and giving an average spread over a much longer period.
Everyone buys petrol in litres now, so to work out mpg (mixing your metric and imperial units).
Taking 1 imp gallon = 4.55 litres ( 1 US gallon = 3.8 litres)
Say you did 300 miles and then top up with 28 litres ( 28 / 4.55 = 6.15 imp galls)
divide 300 miles by 6.15 gals = 48.8 miles per gallon.