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@DaveF1006 your comments were helpful. I did finally resolve this issue slightly differently.

On the TT EasyStep question shown below, I put in the difference between the Empower 401(k) 1099-R Box 1 MINUS Box 2a ( ie,  the gross distribution minus the taxable amount).

 Now my 1040 line 5a & 5b match the 1099-R Box 1 Gross & Box 2a Taxable Amount respectively.