(a) The Agency shall adopt regulations specifying the percentage of the outstanding principal indebtedness which may be insured under this Act with respect to each category of mortgages authorized to be insured under this Act. Mortgages may be insured only for such percentage of the amount of risk as the Agency determines is necessary to induce qualified lending institutions to make such mortgages for the purposes specified in this Act.(b) To qualify for mortgage insurance, a borrower shall be, or by reason of a mortgage insured pursuant to this Act shall become, the owner of the residential structure of more than 5 units for which an insured mortgage is authorized and shall be able to bear the usual expenses of maintaining such structure and repay the mortgage. The Agency may, by rule, establish such additional requirements as it shall deem necessary to accomplish the purposes of this Section.(c) For the purpose of increasing the efficiency and minimizing the cost of the mortgage insurance program, the Agency may insure or issue commitments to insure mortgages upon the certification of an officer of a qualified lending institution that the borrower is qualified for mortgage insurance according to eligibility requirements specified by the rules of the Agency. However, the Agency shall authorize such certification only for mortgages with respect to which the qualified lending institution retains a substantial portion of the total risk.(d) Maximum mortgage amounts shall be established by rule by the Agency, consistent with the findings and declarations in this Act.