(d) Increased mortgage interest costs. The displacing Agency shall determine the factors to be used in computing the amount to be paid to a displaced person under paragraph (b)(2) of this section. The payment for increased mortgage interest cost shall be the amount which will reduce the mortgage balance on a new mortgage to an amount which could be amortized with the same monthly payment for principal and interest as that for the mortgage(s) on the displacement dwelling. In addition, payments shall include other debt service costs, if not paid as incidental costs, and shall be based only on bona fide mortgages that were valid liens on the displacement dwelling for at least 180 days prior to the initiation of negotiations. Paragraphs (d)(1) through (d)(5) of this section shall apply to the computation of the increased mortgage interest costs payment, which payment shall be contingent upon a mortgage being placed on the replacement dwelling.