There is a thing though most people overlook by considering the time, but not also the space. Marriages and children have a tendency of localization, restrictions (geographical, social, cultural, economical) narrow the spreadout of genes. Of course, exceptions may break such barriers.
Also there are famous bloodlines known to be extinct (well, some illegitimate childs may prolong the bloodline beyond the official acts). In these cases no optimistic probability can make one a descendant of them