What is the California maximum percent of AGI that a donation of stock to a qualified charity can be deducted?

California limit is 50%. 

The federal government has "50% organizations" but also "30% limit" and "20% limit" organizations (and in fact, for some donations to a 50% limit organization, the actual limit is 60%).

See publication 526 for definitions of the types of charities and their limits. 

https://www.irs.gov/pub/irs-pdf/p526.pdf