Mont. Code § 85-7-2132
When straight maturity bonds are issued, the board of commissioners of the district shall create and maintain a sinking fund sufficient to pay and discharge the bonds at maturity. If the bonds are issued for 20 years or less, there shall be annually levied for the sinking fund a special tax or assessment sufficient to produce a net amount represented by the quotient found by dividing the aggregate amount of the principal of the bonds by the number of years the bonds have to run; but if the bonds are issued for more than 20 years, it is not necessary to levy a special tax or assessment for sinking fund until the 20th year prior to the maturity of the bonds, at which time and each year thereafter there shall be levied and collected a special tax or assessment sufficient to produce a net sum equal to one-twentieth part of the aggregate amount of the principal of the bonds.
§ 85-7-2132, MCA