Mich. Comp. Laws § 208.1303

Sec. 303. (1) Except as otherwise provided in subsection (2) and section 311, the sales factor is a fraction, the numerator of which is the total sales of the taxpayer in this state during the tax year and the denominator of which is the total sales of the taxpayer everywhere during the tax year. (2) Except as otherwise provided under this subsection, for a taxpayer that is a unitary business group, sales include sales in this state of every person included in the unitary business group without regard to whether the person has nexus in this state. Sales between persons included in a unitary business group must be eliminated in calculating the sales factor.