Your Roth IRA contribution would not factor into your insurance premium tax credit. You may be confusing this with a Roth IRA conversion, wherein you transfer funds from a traditional IRA into a Roth IRA. Such a transfer can result in taxable income that can affect the income considered for market plan premium credits.
There would be no special treatment in calculating your premium tax credit as a result of you making a Roth IRA contribution during the year, except in the case that you did a Roth IRA conversion, in which case your income may be higher by the amount of the transfer. Assuming you entered your Form 1099-R reporting the conversion distribution, your premium tax credit should be accurately calculated on your tax return.
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