(1) A small harvester may elect to calculate the tax imposed by this chapter in the manner provided in this section.
(2) Timber shall be considered harvested at the time when in the ordinary course of business the quantity thereof by species is first definitely determined. The amount harvested shall be determined by the Scribner Decimal C Scale or other prevalent measuring practice adjusted to arrive at substantially equivalent measurements, as approved by the department of revenue.
(3) Timber values shall be determined by either of the following methods, whichever is most appropriate to the circumstances of the harvest:
(a) When standing timber is sold on the stump, the taxable value is the actual gross receipts received by the landowner from the sale of the standing timber.
(b) When timber is sold after it has been harvested, the taxable value is the actual gross receipts from sale of the harvested timber minus the costs of harvesting and marketing the timber. When the taxpayer is unable to provide documented proof of harvesting and marketing costs, this deduction for harvesting and marketing costs shall be a percentage of the gross receipts from sale of the harvested timber as determined by the department of revenue but in no case less than twenty-five percent.
(4) The department of revenue shall prescribe a short filing form which shall be as simple as possible.