Cutting to the chase, the only way to avoid the problem with a highly-compensated employee is for the employer to increase the employee's total compensation by the amount that would be necessary for the employee to pay the added taxes on the cost of the health insurance that must be included in gross income.
Your health care plan costs $500 per month. The employer agrees to pay the entire cost, but other employees must make a $100 per month copay. You are a highly-compensated employee, therefore this violates IRC 105(b) and the $500 X 12 = $6,000 must be included in your gross W-2 income for the tax year. The marginal tax rate for federal and state income tax in your circumstances is 35% federal and 9.3% state = 44.3%. $6,000 X .44.3 = $2,658 income taxes, so your employer must add another $3,344 to your gross income to make up for the increase in your wages created by the violation of IRC 105(b).
Note: The amount that must be added to your income is greater than what the marginal tax rate suggests, because every increased dollar of income adds additional taxes; the actual formula is: annual_heathcare_cost * exp(marginal_federal_rate + marginal_state_rate).
Hope this helps.
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