It depends on how you answer certain questions. In the rental property section under your Property Profile, indicate that you rented out part of your home.
On the page, Let Us Calculate Your Expense Deductions for You, a yes answer will allow you to enter the entire mortgage expense and TurboTax will do the math for you based on the rental % of your home. If you choose this method, then you wouldn't enter the deductible personal portion again. TurboTax would transfer the personal portion to schedule A.
A no answer, would require you to do the math and split the rental/personal portions. Enter the rental share under the rental, then navigate to the Your Home section to enter the personal share of interest.
The page 'Let us calculate your expense deductions for You', is still there in the rental section. To enter the percentage, figure the ratio by taking the square foot of rented part of home rented/to the square foot of entire home.
Ex: rented footage is 140 square feet and total property is 1400 square feet. 140/1400 = 10%
Enter 10% in the rental use percentage box and continue to enter your expenses.