I have never quite understood the notion of "how many amps does it take to support the voltage"
I set the charger voltage and how many amps I get depends on the battery at anything below charger max amps limit. I have no way to change how many amps that is except to change the voltage on the charger which also changes the voltage on the battery.
I don't know how you can see voltage falling off when the charger is set to that constant voltage.
I can see how the "marker" for when to stop the gen could change when you use the amps amount as they taper, like my 5 amps per 110AH = 90% SOC at 14.5v rule of thumb. 220AH would be at 10 amps. If the battery has lost AH, then the amps at 90% would be lower than usual. But that is still at 14.5 volts.