If you go to the child support website they have an online calculator where you can punch in the numbers of how much you earn, how much he earns and how often you have the children overnight. At the end you can then do a what if scenario to see how earnings and time with change the payments.

Reality is you could end up paying him child support if you earn more than him and have significant shared custody. CSA I believe wont care who has taken care of the children in the past just what the shared care arrangement is now.