If you are ordered to pay a certain amount in child support, and you fail to pay that amount, you can certainly be taken to court for underpayment (or through Illinois child support enforcement - I'm not sure about your individual circumstances).
Working this out with the custodial parent would be much better than simply deciding yourself how much child support you owe. I recommend consulting with an attorney to determine the right way to go about this given your circumstances.
This post is for informational purposes only. This post should not be construed as legal advice or as having created an attorney/client relationship. For specific legal advice, you should consult with a qualified, licensed attorney in your jurisdiction.