Being from Wisconsin Dells (only around an hour away from Green Lake) and now living in STL, I feel I'm pretty qualified to answer... we usually stop in Oglesby, IL to get gas and switch drivers (and sometimes eat). There are a few gas stations as well as several fast food eateries to choose from. It's near Peru & LaSalle, first exit south of the river (whose name escapes me). I think this is about the half way point. We'll be making the drive this weekend, in fact!
Streator, Illinois, a few miles east of I-39 is almost exactly halfway.
Washington DC
After looking at coordinates, approximately Dubuque, Iowa.
St. Charles, Missouri is a good halfway point; just west of St. Louis.
Eureka Center, Wisconsin is the halfway point.
The halfway point between Chicago, Illinois and St. Louis, Missouri is McLean, Illinois according to MeetWays.com.
The halfway point between Lafayette, Georgia and Clinton, Missouri, falls in Charleston, Missouri.
The halfway point between Mt. Pleasant, Iowa, and Branson, Missouri, is about halfway between Fulton and Jefferson City, Missouri.
The halfway point between Wisconsin (WI) and Vermont (VT) is Pepper Pike, Ohio.
Jacksonport, Wisconsin, is the answer.
There is 610 miles between Milwaukee, Wisconsin, and Toronto, Canada. The halfway point between these two cities is Lansing, Michigan.
Saint Charles, Missouri
somewhere in Wisconsin