It is a simple proration ... say you pay $12000 in mortgage interest on the home you convert to a rental say 7/1 ... thus the interest is prorated 50/50 ... $6000 goes to the Sch A and $6000 goes to the Sch E.   Same thing for the RE taxes and the homeowners insurance and any other common expenses. Follow the program interview carefully in this section and understand that once you moved out and made it a rental you had NO personal use days in the year of the conversion... this is the most common error ... READ the screens carefully and go slow.