I did run an empirical test on my Standard Rad battery and Rover 5. I ran full throttle on level roads, stopping every two miles to measure battery voltage after 30 seconds. I went 20 miles before the battery was basically exhausted, so for the most part every mile is 5% of capacity. I based my meter chart on these results.
I like your chart. Your knee at 44 volts(20%) agrees with other curves I've seen. For several months, I amassed empirical data by logging voltage and mileage readings each morning before riding. I've noticed that knee, so now I recharge at that point. From my data, I've noticed another knee: above 50 volts or so, I tend to get a lot more "miles per volt."
Internal resistance may be the problem. You were apparently able to maintain an efficient speed. Around here, I often have to slow to a walk. Perhaps below 50 volts (65% on your curve), battery resistance limits the current the controller can deliver. Acceleration would be slower and I'd spend more time climbing at inefficient speeds.
In particular, a 300-yard grade at 6% leads to my house. The whole block is 600 yards. Historically, the limit was 35, and that was quite safe. There are four small houses on that block, and apparently at least one household with political pull wanted it treated as a private road. About 1990, the town posted it at 16 mph and ordered the town cop to issue tickets. It was expensive for a town of 200 households to have its own cop, but how else could they enforce a law like that? There was a rapid turnover because it was as abusive to cops as to those ticketed.
I used to observe the hill with a stopwatch. In spite of all the tickets, about half the vehicles continued to drive at 35. That meant the cops were profiling. My mother was on the council. I told her the limit was a ridiculous burden to anyone who obeyed it. She ignored me until the day she got a warning. I think the cop was politely making her aware that he'd been told to enforce a ridiculous limit. When she brought it up at the next meeting, councilmen scoffed at her and fired the cop.
A mayor was elected who loved to do favors and inconvenience people. He got only 12% of the vote, but nobody else wanted the job. He had a speed bump put in at the bottom of the grade and another 200 yards from the bottom. Vehicles including his would come to a complete stop for each bump. He didn't mind the inconvenience as long as he was inconveniencing others.
After I installed a layback seat post, my Radrunner was the only vehicle in town that crossed those bumps without slowing. Hitting them at 25 wasn't uncomfortable because my riding position put most of my weight on the pedals. I changed my mind when I got home from the grocery store with cracked eggs. That wasn't so bad, but I was afraid I might arrive with broken spokes the next time. Now I slow to a virtual stop, like everyone else, and climbing that hill is a drag. I should have gotten a mid-drive bike with gears.