A little update. My battery capacity has improved after a few charge cycles. Today I rode 15 miles, and adding that to the 20 miles since the last charge, I've gone 35 miles at which point the battery was down to 46.2V (icon showing 1/4 charge remaining, which seems about right). Topped off, it reaches a bit over 54V. So probably ~42V would be the end. I'm charging it now, though.
I did notice a loss of power when using PAS 3 (set for 0-3) up a pretty steep hill, toward the end of the ride when voltage was somewhat below 48V. By the time I was near the top of the hill I could feel the motor was just not putting out as much help as it had been moments before, and the sound it was making changed pitch. I figure the 750W MXUS motor is probably searching for more amperage to make up for the decline in voltage, and not finding it. (I probably could raise the amp setting in the display, but I figure they set it where they set it for a good reason, so I'll leave it alone.) Thereafter, climbing similar hills in PAS 2 and supplying more leg effort did not yield a similar power loss sensation. It's just a factor I'll take into account in the future.