alex- you say its an 8in drop per mile, but i read its a 19in drop, thats twice as much.
NY times had an article on curvature, read the whole thing
http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9505EEDA1F31E733A25755C0A96F9C946897D6CF&oref=slogin
so chicago is 579ft abouve sea level, the highest building is 110 stories so a "high building" could be about 90 stories which is roughly... 720ft high
so sea level + building height
579 + 720 = 1299
i couldnt find the level of michigan city but if chicago is 579, holland is 662, mill. is 612, than we can assume its about 560-600ft plus they built the observatory on a sand dune
the two cities are about 45mi away from each other, 50 max
the observatories could NOT see each other! so dont give me that 8in per mile garbage, US officers say you fail