Answered The question is are you or the other party seeking child support? Child support can be "reserved", meaning neither party will receive child support until either party petitions for it in the future. This is sometimes common in cases where the parties split custody on a 50/50 basis and neither party is the residential parents. However, when one party does seek child support in such a case, the court may look at your respective incomes and order the party with the higher income to pay 20% of the difference in incomes to the party with the lower income.
Answered I don't know how many children are involved, split custody (historically speaking) is generally done when there are many children (3+ in my experience). Regardless, 505 applies to both parents, so either parent is free to petition for support. The amount awarded would be based on the needs of the child(ren) if the state guidelines aren't applicable for some reason.
Whenever you need support outside of the guidelines, you almost always need an attorney to properly petition and prosecute the support case, at least if you want it done properly.
That said, consider talking to the St. Clair County Bar Association for assistance if money is an issue. Best of luck & hope this helps!