Re: Fuel Log
or just go for a longish flight in said aircraft, dip tanks before and after, then take the difference and divide by tacho hours flown. If you do this over a number of "typical" flights you very quickly learn if the "book figure" of fuel burn is on the money or not. Then you can guesstimate fuel burnt based on how many tacho hours you have flown in that flight already and then do a gross error check against the gauges.
Works out pretty accurate at the end of a flight, if you are worried about being within tolerances at the end of a training flight for say CPL they just like you to have tanks within 30mins fuel of each other and also within 30mins of what you think should be left.