With an annuity, only part of the amount you receive will be subject to tax. Whatever amount your mother originally contributed to the annuity has already been tax paid, so the only part you will owe tax on is the interest that has accrued on the account.
You would really need to check with the issuer of the annuity to see how much of that is interest and how much was your mother's original contribution.
But generally speaking, since the entire amount will not be taxable, then having 10% federal withheld should be adequate. I would also suggest having another 3% withheld for CT state taxes. This should be at least a close enough estimate where you would not owe much more at the end of the year and may even get a partial refund. And this is as close of an estimate as I can give you without knowing what your mother's original contribution was to this annuity.
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Thank you Jake