Generally, I feel the calculation method is more accurate, although it's prone to errors of its own, but they tend to equal out over time. For example, if you "top up" tank 1, the performance for that tank will look worse than it should, because you've added more fuel, which you "used" in that tank. Then if you don't top up the next tank, that tank will look all the more impressive, because you didn't put in the same amount of fuel as you burned.

This method pretty much proved to me that to my dealer "a full tank of gas" means "just enough so that all the bars on the gas guage are filled."