2 Answers

The courts usually decide but I would say 1/2 of everything. Food,clothing, health insurance, school supplies. Kids really are not that expensive to raise if you don't buy them everything under the sun. 

I believe child support is based on the income of the non-custodial parent.  It can be modified. YOU don't calculate child support; the court has that authority, as Colleen said.
Many non-custodial parents are dickweeds who work for other dickweeds who will pay them "under the table" so there is no paper trail of income and the child support goes into arrears.
If a custodial parent receives AFDC (aid to families with dependent children) and other social services, a record is meticulously kept so the non-custodial parent can have reimbursement of the assistance added to his debt.  That's the way we do things in California. 
The debts are seldom repaid, but I do know two men who are under court orders to pay their arrears . One's daughter is 24 and the money goes to her mother.