I am a noob, so take what I say with a grain of salt.
I like to be on the ground at 30%. I have logged 14 flights with no crashes, just one unplanned landing on a juniper bush, without penalty. ;>)
Only once did I land with less, 24%, and the red lights were flashing as I was coming in for the landing. I was upset with myself, but I guess that is what noobs occasionally do. I can tell you that after 30%, the charge seems to fall off quite quickly.
Again, my goal is to have her landed by 30%. I consider myself to be a very conservative pilot.
For Fathers Day my bride bought me a Blade Nano QX. That little guy is crazy hard for me to fly. BUT, you can fly it for 6 minutes, and if you run her dry... she crashes well. LOL I like it because I can still fly on windy rainy days, be as reckless and irresponsible as I want and all without penalty. At worst, I'll be out less than $100.
Good fortune and fly safe......