If box 2a of the Form 1099-R shows the correct taxable amount calculated by the payer, you shouldn't be using either the Simplified Rule or the General Rule in TurboTax. You should be telling TurboTax that the amount in box 2a is the taxable amount.
The General Rule is only used for determining the taxable amount of distributions from nonqualified plans and can also be used for distributions from qualified plans where regular distribution began before November 19, 1996. Otherwise, for qualified plans the Simplified Method is used for calculating the taxable amount.
See IRS Pub 939 regarding the use of the General Rule:
https://www.irs.gov/pub/irs-pdf/p939.pdf
