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If the 1099R has listed the taxable amount in 2a then they have already taken into account.
That is the total amount of after-tax contributions you paid to your retirement system while working. Your plan used this to determine the after-tax contribution amount shown in Box 5.
You should not have to do any other calculations if your box 2b shows the taxable amount.
If 2b is checked then you are allowed to count your contributions