Might want to post this on the singleshot thread, as it will get more replies there.
Offhand a midrange sight will probably get you close to 500 yds, and that's farther than most folks feel comfortable taking a shot at game. Of course you didn't mention the rifle type, and some rifles will have lower tangs than others, so on something like a Hepburn the midrange might not get you 500 yds.