Iowa Code § 468.39
At the time of appointing said commissioners, the board shall fix the time within which said assessment, classification, and apportionment shall be made, which may be extended for good cause shown. Within twenty days after their appointment, they shall begin to inspect and classify all the lands within said district, or any change, extension, enlargement, or relocation thereof in tracts of forty acres or less according to the legal or recognized subdivisions, in a graduated scale of benefits to be numbered according to the benefit to be received by each of such tracts from such improvement, and pursue said work continuously until completed and, when completed, shall make a full, accurate, and detailed report thereof and file the same with the auditor. The lands receiving the greatest benefit shall be marked on a scale of one hundred, and those benefited in a less degree with such percentage of one hundred as the benefits received bear in proportion thereto. They shall also make an equitable apportionment of the costs, expenses, fees, and damages computed on the basis of the percentages fixed.
Iowa Code § 468.39
Referred to in §468.8, 468.13, 468.184