New York State law has a formula for the determination of "basic child support". The formula is generally applied to income from all sources up to and often beyond the "cap" set forth in the formula. The most common deductions from income are the payments for spousal support, child support for other children pursuant to an order or proper agreement, FICA/Medicare taxes and unreimbursed business expenses. The income is then multiplied by a percentage based upon how many children are to be supported. This is a very general and very basic explanation of "basic child support". There is much more to this than the above broad overview.
However, in addition to "basic child support" there are child support add-ons. The parents are often required to maintain and pay for health insurance premiums for the children and contribute to uninsured/unreimbursed/co-pay expenses of the children. The amount of contribution is usually based upon a comparison and pro rata of the income of the parents. This is so important because in addition to contribution for the insurance premiums there is contribution to the expenses not covered by insurance. Often the agreement or court order will limit these expenses to "participating or in network providers" and also prohibit cosmetic or elective procedures unless mutually agreed or in the event of an emergency.
Child care is the other major add-on that is mandatory. If child care expenses are incurred so that a parent can either go to work, search for work or continue their education, child care expenses will also be shared in proportion to the incomes of the parents. If there are several young children the costs of child care, if throughout the work day, can often equal or exceed the income of a person who is in an entry level position. Thus, the cost of child care can be a factor in whether a parent seeks employment.
Discretionary add-ons are extra-curricular activities such as competitive sports, music lessons, dance, gymnastics, and educational expenses such as tutoring or college entrance exam prep classes. These expenses are not always ordered by the court and will be much more "fact sensitive" with analysis of the income of the parents, the education of the parents and the past practices for this or other children of the parties.
In my opinion, it is important to have the advice of an experienced matrimonial attorney when addressing these issues as there are many exceptions, nuances and discretion in applying what at times may seem to be a straight forward question of how much child support should be paid.