Except as provided in s. 71.45 (2), “Wisconsin net income”, for corporations engaged in business wholly within this state, means net income and, for corporations engaged in business both within and outside this state, means the amount assigned to this state under s. 71.25 (6), (10) (c) or (13) or by a separate accounting or allocation, if allowed under s. 71.25 (6), or by another method approved under s. 71.25 (12).