Who is the custodial parent for a child over 18?

Generally, you should look to where the child/18yo is living the majority of time. If the child/18yo isn't living with either parent, then the child might be emancipated (and no support would be needed, depending on the facts - college is not an emancipation event). If it isn't clear, or both parents have exactly 50/50 access, then the law says the "monied spouse" pays (so the person who earns more pays child support). In a case like that, it would be reasonable to deviate from the CSSA amounts.  So, as you can see from my answer there are many factors that may be dependant on what's happening in your case. You should talk to a lawyer about the case to know for sure. Good luck.