How does turbotax determine the taxable amount of my IRA distribution that is a rollover?

A rollover from a before tax account to another before tax account is not taxable.   That would be determined either by the 1099-R box 7 code or your telling TurboTax in the interview that it was rolled over.

Do you have a specific question?

If you took a distribution from a IRA that was rolled over from a 401K it is probably 100% taxable.   Unless you have a basis in the IRA or 401K or it is a ROTH account.  The taxable amount is based on your answers to the questions.  Did you get a 1099R?  What code is in box 7?

I read the question that it was the distribution that was rolled over, not that it was a distribution from a previous rollover.

Perhaps the poster can clarity exactally what is being asked.