Why not talk to a lawyer?
I paid about $150 to get an hour of a a good attorney's time to answer all of my questions. It was well worth the money because since I was paying I got real answers instead of a sales pitch. The idea was that if you need to fight this, you would then have an attorney. I think usually just talking to an attorney helps. Make sure you figure out out of your questions before you go to maximize the time.
I live in Massachusetts, but I am sure you could do the same in California. The lawyer can tell you for sure if the child support you are getting is fair, and exactly how much you could expect if you went to court. Especially with the additional complications (i.e. other children, marriage and 50/50 split) an online calculator is probably not going to be very accurate.
Just being able to say "I spoke to an attorney and you are full of crap" is worth the $150.