If you have a pending modification at the time the guidelines change, the new guidelines will apply. Normally, child support is calculated as a class order, however, since one child will soon no longer be eligible for support, it would save time to do it as a per child order to avoid another modification. If your ex won't agree to this, you can make this argument to the judge and let him or her decide. Although courts generally follow the guidelines and do class orders, they are permitted to do it differently if they feel it is apporopriate.
The guidelines used will be whichever are in effect at the time the court issues its order.
I have never heard of using a "per child, per month" order of this type. But in any case, the order will specify that support ends or modifies when a child becomes ineligible. It would certainly be wise for you to have the order spell out what support will be both before and after your eldest turns 21.
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