10% might be arbitrary but it seems to be a number that people have found to be reasonable for over 2000 years, so it is well tested. If you go too low then les

10% might be arbitrary but it seems to be a number that people have found to be reasonable for over 2000 years, so it is well tested. If you go too low then less money goes to charity. Too high and people brush it off as unreasonable. On a side note, Judaism has a 10% min but also a 20% max (but which doesn’t necessarily apply to the very wealthy).