How to get an uncontested divorce and calculate child support?

After 10 years of marriage and two kids, my wife and I are splitting up. We are still friendly and have agreed on all our marital issues except for child support and want an uncontested divorce. I make more than she does but we are going to have 50/50 custody of the kids. I’ve tried using the online child support calculator, but one of my kids has substantial medical expenses and we aren’t sure how to divide those expenses or include them in child support. Also, I pay support for another child from a prior relationship and I’m wondering if I get any credit for those payments against whatever I owe for my kids with my wife. What do I need to do to start the uncontested divorce process in LA and are we able to get one if we still aren’t sure about child support? How do we figure out the right amount of child support based on our circumstances and joint custody?