Wis. Stat. § 79.006
The information needed for the determinations under this chapter shall be calculated as follows: for those years for which the necessary data does not exist, the data for the new municipality and the municipality from which it was formed shall be combined and the sum shall be apportioned to each municipality in proportion to its respective full value in the first year of assessment of the new municipality; if the municipality is formed before April 1, the department of administration shall certify the population of the newly created municipality, and the corrected population of the municipality from which it was formed, and those figures shall be used for distributions under this chapter in the year after the incorporation.
Wis. Stat. § 79.006