if i can't find an answer rooted in nutrition science, i'm going to take a guess based on mimicking ocean water. while i understand that we can't drink saltwater, it's also the case that the salinity of our blood is about the same as seawater's, and that's long been thought of as more than coincidence.

that would suggest about 1%.

for the breakfast bowl, i'm currently at about .5%, but i've missed a few sources.