I just filled up again yesterday, dash reading said avg mpg was 19.9. Hand calculation was 19.6, so pretty close there. As I suspected, my mileage had gone down just slightly with more around town driving and less highway driving (my 'average' speed was only 29mph...). Either way, two takeaways for me personally. one, the dash estimated reading, at least for my vehicle and in my recent experience when I have double checked it, is acceptably close for my needs. It has pretty much always been within .3 or .4 mpg, if not closer. Two, I am not a believer in higher octane gas being of any mileage benefit, at least in my scenario with my qx60 anyway.
The last unscientific 'test' I will do here in the next tank or two, as long as I can remember, is to put the driving mode in ECO, and run a couple of tanks on premium and a couple on regular and see what the mileages look like.