We get a lot of chatter about battery usage and battery life around here. Great way to start an argument. LOL.
It's a good idea not to go below 20%. There seems to be a general consensus that putting a load on a battery below this level will have a noticeable effect on its longevity. That and keeping it out in the cold are likely to be bad for it.
I have a 48 volt battery, so the 20% level is at 42 volts. I set the cut-off at 42, but rarely go below 44. Not because of battery life, so much as that's the level where there is very little boost left. I have gone down to 42 2-3 times (owned the bike 2 years), when I was out on a long ride and getting close to home.
So here we are, 2 years and 3000+ miles down the road, and the battery is still doing fine. So it's working for me.
I've noticed that newcomers tend to overthink battery stuff. Get a few hundred miles under your tires and you'll see what happens -- you'll find a groove that works for you and not think about it any more.