calculating child support under IL state law, military child support

750 ICLS 5/505(a)1) Child support is determined by Illinois statutes. Those laws use a percentage of the paying spouse's net income, after certain specified deductions, as the basis for the child support calculation: 1 child = 20%, 2 children = 28%, 3 children = 32%, 4 children = 40%, 5 children = 45% 6 or more is 50%. If the court deviates from these child support guidelines, it must make a written finding why the deviation is appropriate, based on the following laws: The financial resources and needs of the child; The financial resources and needs of the custodial parent; The standard of living the child would have enjoyed had the marriage not been dissolved; The physical and emotional condition of the child, and his educational needs; and
The financial resources and needs of the non-custodial parent.

You do not specify your service, but each service (Army/Navy/Air Force/Marines) have published guidelines that are to be looked at in the absence of a court order. If there is a court order that will take precedence.