So in our hypothetical where he wins statewide with 35% and nobody else gets 15%, we can assume Sanders will be viable in every or almost every congressional district and that he’ll win at least half of the districts. The odds are against four or more candidates hitting the 15% threshold in most districts, and so wherever Sanders wins with 35% or more, he’ll get half the delegates or more. In some districts, he’ll get all the delegates, because he’ll be the only viable one.
Sanders’s best counties four years ago were small rural counties. He beat Hillary Clinton in the 1st Congressional District by 18 points. Assuming there’s some brand loyalty there, it wouldn’t be hard for Sanders to get all four delegates there by being the only one above 15%.
Sanders could definitely get half of the 271 district-level delegates, which, combined with the 144 statewide delegates, could give him about 280 of California’s 415 pledged delegates. If the non-Sanders vote is more or less evenly split, and Warren is picking up a few congressional district delegates here, while Biden is picking up a few there, and Buttigieg is grabbing a few over there, there’s a decent chance the second-place finisher in California gets only 50-75 delegates.