Here is a web site that will be really useful to you in your quest to tighten up those groups. I have been shooting ML rifles for years, and it helped me immensely.
http://www.blackpowderrifleaccuracy.com/
That Pedersoli rifle should shoot the eyes out of a snake at 50 yards, or 100 for that matter.
As for your eyesight, have you considered aperture sights? I am 60 years old, and my eyes are just like yours, fading. I bought a Pedersoli Gibbs rifle last year, and it has a long range soule sight on the tang, and a globe, or aperture sight on the front, with interchangeable apertures. To start with, I was struggling to see very will in some (most) light conditions, then I read about the "Hadley eye cup". It is a disc for the rear sight, and it has 6 different sized apertures on it, allowing you to flip from one to the other. What I found was that I could always find an aperture that allowed me to see the front sight perfectly, and by putting in the correct aperture in the front sight, I could also see the target. It is 1860' technology, but it works excellent! I have always shot with peep sights, but I only recently discovered the adjustable peep. Check out
www.buffaloarm.com. They sell a good variety of sights, some compatible with muzzleloading rifles. Also, check out
http://www.meritcorporation.com/products.html. They sell a sight that works like the aperture on a SLR camera. It's really cool, but not legal in some competitions. I guess the point of all of this is, that there are sights out the for us old guys, and they work! PM me if you want to chat....
Glen