Rule 505.Credits and the net monthly obligation

The gross obligation. Each parent's gross obligation is their primary support obligation plus their standard of living adjustment.

What they keep. From that, each parent retains the primary support allowance for the children of the union in their primary or shared placement; the child care, private school or other primary expenses they themselves claimed; a per capita share of the parents' combined standard of living obligation for the children in their placement; and the parenting time adjustment where it applies.

This is what makes the Formula symmetrical. Both parents are assessed an obligation for their children; each keeps the part attributable to the children living with them; and what changes hands is the difference. Nobody pays support to themselves.

The parenting time adjustment. Where a child spends an average of more than 79 but fewer than 164 annual overnights with the parent from whom support is sought, that parent retains a percentage of the primary support allowance allocable to that child and of the combined standard of living obligation: 10% for 80 to 124 overnights, and 30% for 125 to 163.

The reasoning is straightforward. A parent who has a child roughly a third of the time is buying food, running heat and keeping a bedroom, and at 164 overnights the arrangement becomes shared placement under Rule 503 instead.

Proving the overnights. By court order, written agreement, previous finding, or other clear and convincing evidence. A party asserting a different number than the order, agreement or previous finding shows carries the burden. Modest or temporary departures from the schedule prompt no adjustment and no rebuttal of the Formula — a missed weekend is not a support case. Where the arrangement is complex, with children in different ranges, the percentages are averaged.