The corporation shall fix insurance premiums for the insurance of mortgage or loan payments under the provisions of this chapter, such premiums to be computed as a percentage of the principal of the insured indebtedness outstanding at the beginning of each mortgage or loan year. Such insurance premiums shall not be more than two per cent per year of such insured indebtedness outstanding at the beginning of the relevant mortgage or loan year and shall be calculated on the basis of all pertinent, available data. Such premiums shall be payable by the mortgagors or the mortgagees in such manner as is prescribed by the corporation. The amount of premium need not be uniform among the various loans insured.