Vin = Vref*adcValue/1024
Just a knit pick but this is a discussion about calibration: FSD is 1023 and corresponds to the Aref voltage.
I'm pretty sure the data sheet indicates dividing by 1023.
The doc says you can sample channel 9 for Aref and 15 for ground (though I have not tried it) I have started to change the pin=(pin & 7) lines to make this possible.
I also did a quick "hobby" quality linearity etc. check as a first test at intervals of 1/6 fsd. The lowest was off by 2LSB on the first chip. Since I had a spare Atmel , I swapped it into the same board and only got 1LSB error on the same test.
Seems select on test can be worth while. Save the less accurate chips for non analogue uses.
Noise and stability seemed excellent. sampling two channels connected to lead acid batteries ( via a preamp board ! ) did not show one bit of change (ie zero LSB movement).
Having provided the Arduino with a bandgap ref of 5.00V as a reference my readings were
# sample 125/128
ch_0 = 0 = 0.0000000000 current=0.0000000000
ch_1 = 825 = 26.1755523681 power=0.0000000000
ch_2 = 743 = 12.9945077896 power=0.0000000000
ch_3 = 0 = 0.0000000000
ch_4 = 0 = 0.0000000000
ch_5 = 0 = 0.0000000000
checking ch2 with a good quality (3,5 digit accuracy) DVM showed it flickering between 12.99 and 13.00V
So it looks like I have a good chip but I'm also favourably impressed with the hardware design of the Uno.