Hers and your monthly expenses are not usually factored into the child support calculation. Your respective incomes and percentage of custodial time are factored in. Plus, you get an off-set for the health insurance premiums and you should get a "hard ship deduction" for the child from another relationship. The issue of her monthly expenses and the fact that someone is paying part of her rent would go more to the issue of spousal support and division of property issues. If you did not receive any deduction for the health insurance premiums or a hardship deduction for your other child, then a mistake has been made. Did you tell the court about the health insurance premiums and your other child at the time of the hearing? Do you know if those items were factored into the child support calculations?