(1) The amount of the gain or loss from the sale of assets used in the regular course of business which is apportioned to this state shall be calculated separately if the taxpayer elects to report the gain on the installment method. The separate calculation shall be made:

(a) for installment sales of Montana assets which generate net gains in excess of $2,500,000; or

(b) installment sales of major assets located outside the state of Montana which generate net gains in excess of $10,000,000.

(2) The separate calculation shall be made as follows:

(a) for purposes of the sales factor the total net gains from the sale shall be included in the sales factor in the year of the sale unless specifically excluded from the sales factor under another part of ARM 42.26.263;

(b) for purposes of calculating apportionable business income from the installment sale, the factors of the year of sale shall be utilized in apportioning the gain regardless of the year in which the income is reported; and

(c) the factors of the year of sale shall be applied to the gain from the sale as it is reported by the taxpayer to determine the amount of business income from the sale which is apportionable to this state, unless the gain is accelerated under ARM 42.26.276.