What if there no primary custodial parent?

odd situation: my son attends private boarding school out of state. he is there 205 nights a year. during summer and holidays he is mainly at his mom’s – 160 nights a year, give or take. The state child support guidelines say: “A parent has primary physical custody of a child if the child lives with that parent for at least 243 nights during the year.” so by that definition, we cannot use any of the worksheets to determine support…

what to do?

This is the type of case where a deviation from the Child Support Guidelines would be appropriate.
In deviation cases the court will hear evidence and make findings regarding the reasonable needs of the child for support based on the circumstances in the particular case.