We cannot do calculations based upon your specific situation, but I can give you some information that should help.
You mentioned that you would like to know about social security with a married filing separately filing status. Married couples get taxed on 50% of their social security when their modified adjusted gross income (MAGI) exceeds $32K and that portion goes to 85% when the MAGI is more than $44K. The MAGI considers non-taxable interest and 50% of your social security. This article explains in more detail. With a hypothetical $6K of income a month, the MAGI would be above $44K, so 85% of the social security income will be taxable, as you assumed.
You can have social security withhold income taxes on your behalf. Here is the information on that. Filling out that form will require you to choose a tax withholding rate. This table will show you your marginal tax rates on your taxable income in 2025. Remember that your taxable income is reduced by your itemized or standard deduction amount (typically whichever is greater). The standard deduction for a married couple is $33200 ($30K + $1600 + $1600 for each person over 65--higher if blind). This tax bracket calculator for 2024 will also get you reasonably close. If your situation is similar to last year, you can also check your TurboTax file to find your marginal rate on your last return (for estimation purposes--if the situation is similar). You can use this calculator to guide you on the employment component of your income.
Keep in mind you are making this change mid-year. If you are significantly under-withheld so far, you may consider making an estimated payment to lower your underpayment penalties (which are calculated based upon time). This page from the IRS explains more and how to make the payment.
Hope this helps.
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Regards,
Karen
TurboTax Expert
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