Are we talking of the same Flat Rock Cemetery? The one I know of is west of Napoleon and south of Milhousen. My wife has several ancestors reposing there. We found it once guided by a cousin. Then last May we stumbled onto it trying to traverse the county on the secondary roads. This is in Ripley County. If that is it:
Goingsouth through Napoleon on Rt 421look for a small, slender white arrow pointing west to Union Flat Rock Baptist Church. Similar signs will guide you there. Hope no pranksters have removed them, for it's easy to get turned around out there. Perhaps some kind Hoosier will give us detailed directions including road numbers. I'd like to have that, too.