The TurboTax program uses three figures to the right of the decimal point for Form 8606, line 10.
TurboTax carries at least 6 decimal places internally, perhaps as a floating-point number with many more places than that, but only displays 5 digits following the decimal point on line 10. This means that the number calculated on line 11 or line 12 might not be exactly the amount displayed on line 10 times the amount on line 8 or line 7, respectively. This is unfortunate, since the calculation might not quite make sense just looking at the numbers on the form.
What TurboTax should be doing is rounding the internal value to the value being displayed on line 10 before multiplying to get the values on line 11 and line 12.
Rounding to 3 decimal places can cause a substantial difference in the reported income. For example, a $100,000 distribution can have up to + or - $50 difference. The IRS allows higher levels of precision, and I guess seems that they set the minimum the 3 to allow easier calculations. This looks like a throwback to the days before calculators and computers. In fact, the way computers handle fixed point multiplication, programmers have to go out of their way to round off at 3 decimal places. I had to add a few steps in my cross checking spreadsheet. Seems archaic to me. BTW, TaxCut does the same thing as TurboTax (e.g. 3 decimal places).