This 8" bull should work from 100 to 150 yards. Staple it onto a large piece of map-paper or butcher paper.
http://www.targetz.com/targetzlib/10094.pdfThe whole idea is to use an aiming point that will allow you to shoot a group - then adjust the sights to make that group in the middle.
Hint - the smaller the aiming point that you can see the sights on, the smaller will be the POTENTIAL for your group.
The aiming point should be small enough, it makes you work to shoot that group. different eyes will need different size - and/or colour of bull. Experiment!
The aiming point could be 6" or 8" and with a good load in a good rifle, you should be able to shoot a 2" or even tighter group with open sights, if you are up to it.
Paining the sight so you can see them, is a good idea, as hanshi does. Whatever works. Apertures certainly help as well. With a good set of apertures
like we use on the later period buffalo rifles, you could shoot as tightly as an inch to 1 1/2"- if the gun and you are up to it, maybe even better.