You would usually determine the percentage by the size of the apartment vs the rest of the building. If there are 2 apartments, for example, and you rent one of them, you would use the square footage of that apartment as a percentage of the total building square footage. You would then apply that percentage to the the mortgage and interest.
Say the rented unit is only 10% of the building, you would allocate 10% of interest and tax to that unit, and the rest to Schedule A.
It's much simpler if all the units are the same size. For example if there are 2 apartments that are pretty much equal, you could just choose to allocate 1/2 of the interest and tax to that rented unit.
