Mileage as a function of $$ spent is relatively unimportant in the scheme of things but it's a very useful number for figuring you range between fuel stops.
For seeing you actual fuel consumption I've found my Silverleaf to be much more accurate than hand calc simply because it tough to know whether or not the tank is actually full and if full just how full.
The Silverleaf tell be that I have gotten an avg of 8.5 since I bought it and it's been 8.5 since the engine was born in 2000. Some legs are 10.0 and some are 7.0, but over the length of each year it' still 8.5.
Do I care if it's really 8.2 or 8.7?? Nope