Mich. Comp. Laws § 141.622

Sec. 22. Second, the taxpayer shall ascertain the percentage which the total compensation paid to employees for work done or for services performed within the city is of the total compensation paid to all the taxpayer's employees within and without the city during the period covered by the return. For allocation purposes, compensation shall be computed on the cash or accrual basis in accordance with the method used in computing the entire net income of the taxpayer. If an employee performs services within and without the city, the following examples are not all inclusive but may serve as a guide for determining the amount to be treated as compensation for services performed within the city: (a) In the case of an employee compensated on a time basis, the proportion of the total amount received by him which his working time within the city is of his total working time. (b) In the case of an employee compensated directly on the volume of business secured by him, such as a salesman on a commission basis, the amount received by him for business attributable to his efforts in the city. (c) In the case of an employee compensated on other results achieved, the proportion of the total compensation received which the value of his services within the city bears to the value of all his services.