You cannot deduct the other 70% of your mortgage interest and real estate tax. You can take 30% (if that is your business % on line 7) of the total amount of mortgage interest and real estate tax you paid on the property where your home office is located. Excess interest and tax would occur if you were itemizing and your deductions for state and local tax and mortgage interest were limited to due the caps on those deductions. In that case, you could take 30% of the excess (i.e. the disallowed amounts).
You cannot deduct the other 70% of your mortgage interest and real estate tax. You can take 30% (if that is your business % on line 7) of the total amount of mortgage interest and real estate tax you paid on the property where your home office is located. Excess interest and tax would occur if you were itemizing and your deductions for state and local tax and mortgage interest were limited to due the caps on those deductions. In that case, you could take 30% of the excess (i.e. the disallowed amounts).