The employer can provide tax-free health insurance, and the employee can pay their share of the premiums pretax, only if the covered people are the employee’s self, spouse, or dependents. Because you are none of these things, the employee’s share of your medical premium is deducted after tax, and the employer share is added to the employee’s taxable income, as if they had received a raise in the amount of the employer’s share of the premium. This is imputed income, and it is subject to federal income tax, state income tax, Social Security withholding, and Medicare withholding.
The extra cost that your ex is paying for your medical coverage, is the after tax payroll deduction for your portion of the insurance, and the income and other taxes that they are paying on the imputed income for the employer’s share.
You will have to ask your ex and their employer to tell you what these dollar amounts are. For example, suppose that single coverage has a $100 per month employee share that is deducted pretax and a $500 per month employer that the employer pays tax-free. Upgrading to single plus one coverage has a $200 employee share and an $800 employer share. In this example, the imputed income would be $300 per month, plus the extra $100 that the employee is paying. And the extra cost to the employee is the $100 per month employee share and the income tax assessed on the $300 per month imputed income employer share.
Once you know the amount of imputed income, I agree that the easiest way to figure out how much extra tax your ex is paying is to ask your ex to calculate their tax return once using their W-2 and a second time by removing the imputed income from the box 1 taxable income, and see what the difference is.
