I would agree with you here. Even if someone tested it and a lower threshold value gave them enough runtime TODAY, that doesn't mean you'll still have enough runtime in the future when the batteries are a little older.
Also true. Every UPS I've seen uses lead-acid gel cell batteries, and one characteristic of lead-acid batteries is they lose a bit of their max capacity every time they're discharged.
My ups is set at a default of 20% from Eaton.
I did a test this afternoon/evening and noticed that the nut package in pfSense uses this 20% value to initiate a graceful shutdown. It left the shutdown procedure far too close to the shut down of the ups.
I ended up adding under Extra arguments to driver:
ignore lb
override.battery.charge.low = 30
essentially this tells the nut package the low battery level is 30% and to initiate a graceful shutdown. No need to adjust the low battery values on the ups.