I'm at 30 for low and 15 for critical. Depending on your flying..ie wind, speed, mode I've seen the percentage drop quickly once you hit the critical point. It's scary. There may be a standard but that's personally how I've set mine.
I’ve never had a problem leaving it at defaults. But I’m relatively conservative about bringing the bird home from any significant distance to allow time:battery for return.
I’d HATE to have it start landing at 15% before I got it back.
Rare that I’m still in the air at 15%, but that’s cutting off more flying time that I’d like.
Better to make sure you’re nearly home by then & have the maneuvering time left for last minute shots or footage before landing.