The amount of tax paid on Schedule A is only the personal use portion. From the Form 8829 Instructions:
Real estate taxes. On line 11, include only the total of your real estate taxes that would be deductible on Schedule A (if you had not used your home for business and treated all your real estate taxes as a personal expense) and qualify as a direct or indirect expense.
Itemized deductions. If you itemize your deductions on Schedule A, be sure to claim only the personal portion of your real estate taxes on Schedule A, not what you treated as the personal portion for purposes of this line 11. For example, if your real estate taxes are an indirect expense and your business percentage on line 7 is 30%, you can claim 70% of your real estate taxes on Schedule A.
https://www.irs.gov/pub/irs-pdf/i8829.pdf
