Hand Calculation Accuracy?

I've read the arguments elsewhere that say the car's computer is measuring the fuel run through the injectors, etc, and that the display is correct. Well, I use it for an approximate mpg guesstimate. But for accuracy, I prefer to add up the gas that I have paid for, which is the important amount for me. In preparing to sell my '06, I recently added up all the gas for which I have paid over 5+ years and 58K miles. It averages out to 54.1 mpg. Interestingly, this was almost identical to the 54.0 I averaged with my '04 over 70K miles. When I get my PiP, I think I will allow the lifetime mpg to accumulate for a long term comparison of accuracy.