Incorrect Answer deleted
Unfortunately this is a situation TurboTax does not support on a single 1099-R.
You must split the 1099-R into two 1099-R's both the same except for the box 1 and 2a amounts and box 4.
On one enter only the amount rolled over in box 1 and 2a. Enter that 1099-R and say you "moved the money" and rolled it all over.
On the 2nd 1099-R enter the amount converted to the Roth in box 1 and 2a and the tax withheld in box 4. Enter that 1099-R and say you "moved the money" then say you did a contamination of things and enter the amount converted (The box 1 - box 4).