Illinois calculates maintenance using a guideline formula based on the couple’s combined net income. You take one-third of the higher-earning spouse’s income and subtract one-quarter of the lower-earning spouse’s income to estimate the payment. In simple terms, divide your income by three, divide your spouse’s by four, and subtract their amount from yours to see what you may owe. An experienced attorney can help you apply the formula correctly, stay within legal limits, and avoid costly mistakes.