Modification of child support

I have three children, and currently pay child support for them, by court order.

In one month, one of the children turns 18, and has already graduated high school. She currently resides with me.

I called child support services asking how to start the process to have child support recalculated, and was told “they will send me forms to determine of a 15% change has occurred”. I just used an online calculator and, because health insurance reimbursement is now considered “income”, even with one child aging out, it appears my payment could possibly only drop 9%.
Firstly, how is one child aging out NOT immediately a reason that a recalculation is allowed. Even if it’s $100 a month, I could really use that extra $100 as my expenses have increased even as hers has decreased.
Secondly, the reason there is such a disparity is that I am paying roughly 81% of the entire calculated amount of child support because my ex works minimally, at an entry level job and has never bothered to try to improve. I currently pay her almost as much in child support as she manages to earn.

Is this true, that I may not be allowed to recalculate child support, even with less children factored into the equation?

Child support orders are not permanent and are always subject to modification based on a substantial change in circumstances.You can file a motion to modify child support, citing as the reason that one child has reached the age of majority.

The fifteen percent rule is as follows: There is a presumption that a substantial change in circumstances exists if an order is at least three years old and there is a 15% deviation from the amount awarded under your current order and the amount that would be ordered currently. The forms ask if there is a 15% deviation because that deviation alone will satisfy the “substantial change in circumstances” requirement.

But you always have the ability to file a motion seeking to modify based on a substantial change in circumstances (i.e., child reaching majority, you lost your job, etc.).

I presented the reasoning you stated to the child support worker, and she stated that while a child “aging out” of the support order was a qualifying event, the calculations would still need to indicate a 15% change before she would authorize a recalculation and modification of the child support order. Personally, with as much as comes out of my salary anyway, if the difference is $50 a month, well… that’s an extra $50 I didn’t have…

Is the child support worker the gateway to modification of the order? She doesn’t seem willing to process anything unless it’s a 15% change, no matter what the reason.