My understanding of "Finish a Tomb" was when you reached a room where the sounds played and the Message at the top right of the screen fired (saying "Seabody has finished the Legendary Forgotten Burial Mound" or similar). That's always fulfilled this wish for me.
It would make sense, the BuyDebug Tomb Marker Object has an option to set a room as the end of the tomb and as far as I know, that's how EA set up their tombs.