It is based upon the time the child is in the parent's care regardless if they are actually there or not. Sometimes the child with siblings or other relatives and the parent has to work and that is acceptable.
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Dad would need a court order indicating that he has a certain percentage of custodial time with the son for it to be factored into the calculation. Child support calculations take into account each parent's income and the percentage of time each parent has custody of the child. Visitation is not custody. In your case, Dad could argue that the weekend your son spends at Grandma's (the same house in which Dad lives) constitutes custody. He could, for example, claim that your son spends 2 days each week (8 days per month) living at his home and that the calculation should take that into account. 8 days per month/96 days of the year is about 20% custody.
Most likely, your husband will be given credit for the weekends, even if he leaves the child with grandma all weekend. As a member of father's family, grandma gets to have time with your son. Your time share is roughly 23 percent. Depending on your judge, you may be able to get the court to give Dad less time with your son on weekends if he in fact doesn't bother to see him. But you need to prove that to the judge. Also, the judge probably will consider allowing gradnma some time, unless there is something wrong with her.
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