How does a Court come up with child support amounts in NJ?

Judges (and lawyers in general) are not renowned for their ability to do mathematical gymnastics in their head. Instead, what they use are these long, drawn out documents called the Child Support Guidelines. In short, based on whatever income you are making, they tell the judge what the proper amount should be. There is little or no guesswork involved. Theoretically, the judge could award a different amount, but since this makes it so much easier, and the documents are approved by the Supreme Court, judges rarely deviate from them.