Green Bay is only a degree west of the central meridian of 87 degrees, and the formula for a degree of longitude is 111 kilometers times the cosine of latitude. In this case that degree translates to only 79 kilometers from the meridian. Since Zone 16 is an even number, the second letter of the digraph will begin with F at the equator. The zone extends from 12-18 degrees east, so the central meridian is 15 east. 52.5 degrees is about 5830 km.
Older maps have a variety of grid systems and lettering schemes.
We'll ignore the way NATO maps handle the poles for this purpose. The central meridian is 3W. Since all the north-south grid lines are parallel to 3W, they will make an angle of 3 degrees with the bounding meridians. As we go toward the equator, the angle gets less. Thus, the maximum difference between grid north and true north is 3 degrees.