Terry,
Look at the references about each mill in the online Ramsay archive, and use those as clues to locate each mill. If it were me, I would study the neighbors' names around Lewis Bohanan in the 1860 census, then find at least a few of the same neighbors in the 1870 census, so you can pinpoint the exact neighborhood for Bohanan's mill. You may have to use process of elimination to calculate which of the five mills was closest to Bohanan, then look for the tell-tale occupation of "miller" listed in each census by those men's names. I noticed that some mills referenced one of the older histories of Laclede County--those published in the 1880's. I think these are now online, too, saving you a trip to the library. Look up those references and take note of the mill owners names, and maybe the old Laclede County records that Ramsay mentions. I know this method sounds tedious, but it often works for me when I run out of other methods to attach a person to a place.
Bruce