Michigan Compiled Laws, Chapter 206. Income Tax Act of 1967 § 206.657

Sec. 657. (1) Except as otherwise provided under this chapter, the tax base of a financial institution whose business activities are confined solely to this state shall be allocated to this state. The tax base of a financial institution whose business activities are subject to tax both within and outside of this state shall be apportioned to this state by multiplying the tax base by the gross business factor.

(2) A financial institution whose business activities are subject to tax both within and outside of this state is subject to tax in another state in either of the following circumstances:

(3) Except as otherwise provided in subsection (4), the gross business factor is a fraction, the numerator of which is the total gross business of the financial institution in this state during the tax year and the denominator of which is the total gross business of the financial institution everywhere during the tax year.

(4) Except as otherwise provided under this subsection, for a financial institution that is included in a unitary business group, gross business includes gross business in this state of every financial institution included in the unitary business group without regard to whether the financial institution has nexus in this state. Gross business between financial institutions included in a unitary business group must be eliminated in calculating the gross business factor.