The easiest way to use the percentage method is to use the party’s gross income to child support ratio. For example: if a party’s gross income is $2,000 per month, and their court ordered child support obligation is $500 per month, the child support obligation represents 25% of his or her gross income. That party would be responsible to pay 25% of any non-recurring or irregular income as child support.
There are arguments to be made that the percentage should be based on net income to child support as the non-recurring or irregular income could be taxed at a different level than the party’s regular income. In the end, this is a matter to be left to the judge’s discretion. I would look at all possible scenarios so you aren’t surprised by any arguments that may be made by the other party.