Do not enter your Form 1098 information in the Personal section of the TurboTax interview. Enter that information only in the Business section, as it appears you successfully did, when working on your home office deduction.
Based on your inputs, TurboTax will correctly determine the percentage of use for the home office, charge the proportionate amount of mortgage interest, property tax, insurance, and utilities to the home office, and transfer the remainder of the mortgage interest and property tax to your itemized deductions on Schedule A.
If the mortgage interest and property tax from Form 1098 have also been entered in the Personal section, there will be a double counting of those deductions. This may be remedied by returning to the Personal section of the interview, choosing "I'll choose what I work on," clicking the Update button next to "Mortgage Interest, Refinancing, and Insurance," locating the Form 1098, and clicking Delete.
