(1) The department shall determine the contract sales price of the coal immediately after the point the coal is prepared for shipment to the purchaser, as defined in 15-35-102, MCA. To arrive at FOB mine price any shipping or any other expenses incurred after the coal is prepared for shipment may be excluded from the contract revenue. The contract sales price will be determined by deducting from FOB mine price or a value imputed by the department:

(a) the allowance for federal, state, and Indian royalties; and

(b) the amounts charged to the purchaser to pay taxes on production.

(2) In computing production taxes the operator may include that amount which he expects to pay or the amount charged to the purchaser. If the taxes actually paid on the production are more or less than the production taxes deducted and affect the contract sales price, the difference shall be an adjustment in production taxes deducted for the following year.

(3) Contract sales price should be computed for each contract individually with the exception of those contracts for which the department imputes value. The resource indemnity trust tax and the gross proceeds tax deductions shall be the actual amount charged to the purchaser.