The authority shall fix mortgage insurance premiums for the insurance of mortgage payments under the provisions of this chapter. Such premiums shall be computed as a percentage of the principal of the mortgage outstanding at the beginning of each mortgage year, but shall not be more than one-half of one per cent per year of such principal amount. The amount of premium need not be uniform for all insured loans. Such premiums shall be payable by mortgagors or mortgagees in such manner as prescribed by the authority.