Actually, the rules are pretty straightforward. For joint custody, it is the "Wright v. Osburn offset test," moderated by the presumptive maximums, all explained at length on our Child Custody page, and in the Rivero case, which is posted on that page. See: http://www.willicklawgroup.com/child_support.
If that explanation is not sufficient, or you need advice as to how it will be applied, you should probably seek consultation with an appropriate family law specialist. If you decide you would like to discuss this with one of the attorneys of this firm, please take a look at our consultation policies, procedures, and costs, at http://www.willicklawgroup.com/consultation_policies.