TT2018, Outstanding Mortgage Amount ~ Do I enter all or half if part is rentals?

This year for what I can recall is the first time.... TT has asked for the full amount of my outstanding mortgage amount as of Jan 1.  Do I enter the full amount as shown on my 1098 form because 1/2 of the building is rental property and I only claim 1/2 as personal and 1/2 as rental expenses.  The amount is less than 1 Million so does it matter?