How is child support calculated when parents share parenting equally?

Under section 9 of the Federal Child Support Guidelines, the court may adjust child support in a situation in which the parents share the children on an approximately equal basis (generally understood that each of the parents have the child(ren) at least 40% of the time) so that the amount the parent with the larger income pays is set-off by the amount that the other parent would pay. Note that while this is often done, it is not mandatory and the court has the discretion to find that it would not apply a lower set-off or even no set-off at all.