Arithmetic average.

Except as provided in sub. (3d), the arithmetic average of the 2 percentages referred to in sub. (3) shall be applied to the net income figure arrived at by the successive application of sub. (2) (a) and (b) with respect to Wisconsin insurers to which sub. (2) (a) and (b) applies and which have received premiums, other than life insurance premiums, written for insurance on property or risks resident, located or to be performed outside this state, to arrive at Wisconsin income constituting the measure of the franchise tax.