It is my understanding that British double rifles were initially shot and the sights regulated at the distance to be marked on the standing leaf. They were then checked at the distances to be marked on the folding leaves, and the sights adjusted by filing if necessary.

Many early black-powder big-bore doubles will have the standing leaf sighted for 60 yds, since the trajectory of those big lead slugs was considered 'flat' out to that distance. My AH .577 Express (much 'flatter shooting' than the bore-guns!) has a standing leaf marked 80 yds, and folding leaves for 150 and 220 yds. ...and you can bet the sights were filed for accurate shooting at those distances!