Michigan Compiled Laws, Chapter 600. Revised Judicature Act of 1961 § 600.1311

(a) Add the number of jurors the judge has estimated will be needed to the number that experience has shown will be eliminated because of disqualification or exemption. Example: If the judge estimates 100 jurors will be needed and the board has found that to select finally 100 jurors, 50 persons will usually be found to be exempt or disqualified, including those who have moved from the county or died, the board shall add 50 to the 100.

(b) Divide the number equal to the total number of names which appear on the list received pursuant to section 1310  1 by the result, obtaining the nearest integral quotient. Example: If there are 50,000 names on the combined list, divide 50,000 by 150.

(c) The result is the key number for the period for which jurors are to be selected. Example: 50,000 divided by 150 equals 333- 1/3, so 333 would be the key number in the example.