Wis. Admin. Code Department of Revenue Tax 2.45
When the business of any person, other than an interstate professional sports club or "financial organization" or "public utility," as defined in s. 71.25(10), Stats., within Wisconsin is an integral part of a unitary business conducted within and without Wisconsin, but because of unusual or unique circumstances the portion of the income of the person derived from business transacted in Wisconsin cannot be ascertained with reasonable certainty by use of the apportionment formula provided in s. 71.25(6), Stats., or by separate accounting in view of the unitary nature of the business, the department will substitute in the place of some or all of the statutory apportionment factors another factor or other factors as will reasonably apportion to Wisconsin the business income properly assignable to Wisconsin. In any case in which an apportionment of business income is made pursuant to this regulation the taxpayer, at the time of the assessment, will be apprised of the factors used in the formula adopted.
Wis. Admin. Code Department of Revenue Tax 2.45
Section Tax 2.45 interpretss. 71.25(12), Stats.