Your question is confusing. Why are you paying an additional $250.00 that was not ordered? Those extra things that you did are nice, but for purposes of child support do not equate to "support." Also, visitation and support are two different things and you seem to be melding the two issues some how. If the state sent you a child support modification sheet in the mail (as they do from time to time), fill it out and send it back as they ask. If there is a motion you received from the child's mother, then you still need to respond to the motion affirming or denying the allegations. To that point, if you really don't know what to do- - hire an attorney. Who can handle all of this for you.
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Looks like your financial situation improved and since you were paying extra money to your ex and paying expenses, she decided not to get a CS Modification at that time.
In Illinois child support is usually paid as a percentage of "take home pay".
However, the court can deviate based on circumstances.
Since you have the child so much of the time, that should be a reason.
As to custody, it really depends on a lot of factors.
How old is she.
Does she have other brothers and sisters.
Where does she go to school.
On and on.
In the end the judge decides custody on the basis of "best interests of the child".
take all your court papers and the ones you recently received to a lawyer. if your parenting time has increased substantially since you were first ordered to pay support, you may get the support reduced but there are some appellate court cases that say that support is not tied to parenting time. there are some that say the reverse.
it may not make sense to file for joint custody. sole custody for you may be what to ask for and then settle for joint. or maybe not ask for custody at all.
take all your court papers to a lawyer.
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