(1) A joint owner will be eligible for the tax credit provided according to 15-35-202, MCA, if the joint owner's total purchases OF MONTANA coal, including the joint owner's share of the coal consumed by the jointly owned coal using facility, exceed the joint owner's base consumption level.   A joint owner's share of coal consumed by a jointly owned facility will normally be determined for purposes of the tax credit computation in the same manner used to determine the base consumption level.   However, if the department determines that the method used to determine a joint owner's base consumption level no longer properly reflects a joint owner's share of current level consumption, the method used to determine a joint owner's current consumption share will be changed.   However, once determined, a joint owner's base consumption level will not be modified except as provided under ARM 42.25.1703.