I would do that 20 hour test but I don't understand what happens if the batterie is not fully discharged when it reaches the 20 hours.
Then you should have used heavier current! That's a paradox with this method.
But, by plotting many curves you can approximate some current that will fully discharge the battery in 20 hours, or whatever rate you are interested in.
And what do I do with that C/20? I would like to put a meter with some leds to have a reference of the battery level.
How I did it for the Space Shuttle in the late 70s was to convert amps to volts using a shunt, and then volts to frequency using an V-F convertor, and then a counter. The count was then proportional to amp-hours used by the battery and so you then knew, in principle, how much charge remained.