YOu can always put a small witness mark, front and back to the magnet to use to position it, and to check its position before each firing to insure it is properly aligned. Many shooters set the slot of the tang screw so it is in alignment with the barrel, and use it to adjust windage positions for their face when they mount the gun to shoot. As for vertical hold, they get used to how much of the front site, or bayonnet lug has to show over the barrel to hit targets at given ranges. Even scratches or light file markings on the back of the barrel will help you check windage on a smootbore when you can employ a traditional rear sight. At ranges out to 50 yds, typical for shooting smoothbores with round ball, you can do amazingly well with just these crude indicators. When you consider how fat, or wide, the bayonnet lug is on a Brown Bess, for instance, you begin to understand that accurate shooting and small groups requires you to mental disect the front sight, and put an imaginary line up the middle of it at the center of the target you are shooting. Then, those scratches, or file marks, or that screw slot seems like a pretty good sight!
