0.015V instead of 0.020V across the one ohm resistor would give you your 12V. (12V/801 Ohms=0.01498 Amps or appx 0.015V across the 1 ohm resistor) And 12 V storage batteries are a bit higher than 12 V when fully charged. How accurate was that 0.02V? How do we know?
How do we know? 0.02 ... probably eyeballed. Easily 0.015.
In the intro Mats said the conductor/800 ohm were to see if the supply was constant current. But it's neither.
What!!! Did M.Y. actually write something I agree with. Amazing.