You need to meet with an attorney to discuss this issue. Your saying your "pretty" sure you did everything by the books may not be correct. Furthermore, you said ncp has never shown "much" interest in legal custody. Has NCP been exercising physical visitation and why do you want supervised visitation?
Obtaining sole legal and physical custody can be very difficult since such an order would have to clearly be "in the best interest of the children". The existence of domestic violence convictions or allegations is certainly a factor the Court will consider when determining what custody arrangement is in the best interest of the children. Speak with an attorney.
While the online child support calculator is helpful an Child support attorney can make sure the number are correct. There may be set-offs that apply or you may have calculated the number incorrectly. If you are currently receiving support from your ex-husband you should meet with an attorney prior to modifying the current order.