Gasman, your own calculations regarding the original example are flawed, so if you use the same flawed methodology in calculating the taxable and nontaxable amounts of your own distribution, you'll get the wrong result. If you entered everything into TurboTax correctly, TurboTax is giving you the correct result an you are not getting taxed twice on anything.
"20% pretax mean 20% post tax." No, 20% pre-tax would mean 80% post-tax. Your traditional IRAs combined contain a certain total amount with some amount being after-tax basis in nondeductible contributions and the rest being pre-tax.
The $11,000 of basis is already part of the $160,697 + $7,947 = $168,644. It makes no sense to add it again. The percentage of basis is $11,000 / $168,644 = 6.5226%. The amount of basis in the distribution is therefore 6.5226% of $7,947, which is $518.35 (rounds to $518 on Form 8606).
Whatever amount of basis is included in your distribution reduces the basis that remains in your IRAs.
If your 2018 Form 8606 shows $1,194 on line 5 and $908 on line 14, it means that $286 of your Roth conversion in 2018 was nontaxable and the rest of the distribution was taxable. One can also infer that your Roth conversion was about 23.95% of the Roth conversion amount plus your 2018 year-end balance in traditional IRAs.
You description of your transactions in 2018 and 2019 seem to suggest that you are thinking that you can convert to Roth just your basis in nondeductible traditional IRA contributions without taking into account the rest of your traditional IRA funds. The law doesn't allow that.
