The mortgage used to purchase your primary residence for the first part of 2018 was $454K. The interest on that until August 31is fully deductible. When it became a rental, it became irrelevant to the calculations for your deductible mortgage interest. For the second part of 2018, your primary residence had a qualifying mortgage of $580K, so the interest on it is fully deductible. You don't add the two together, or use percentages to determine the cap.
