Do you have to pay 20% of income to child support for a second child if the child support order for your first child is far less than 20%
You need to tell us where you're located, but in the meantime let's assume you're in a "straight percentage of NCP's income" state.
CP and NCP of the first child agree to $500/month for child support, around 10% of NCP's income.
NCP has another child.
CP of second child wants guideline support instead. CP will then get 20% of NCP's income minus first obligation. So subtract that $500/month and use the remainder to calculate support.