It's helpful to understand which model your state uses because it will affect the calculation of your child support obligations. Follow these steps to determine the required child support payment of a parent in cases of split custody. In the example above, Paul’s income accounts for 55.01% of the basic child support obligation, while Mary’s income accounts for 44.99% of the basic child support obligation. Looking at the chart, their monthly basic child support obligation is $1,436.00. Child supp...