125 yards max with a .50 and a PRB. With so little powder, i'd stay within 100 yards.

For target practice 200 yards is fine, However, dont expect to get good groupings. My traditions .50 kentucky flintlock with 110 grains FFFg and a PRB will do 2" groups @ 100 yards which is pretty good with the patches burning up like they do

Use a .490 roundball with .018 pillow ticking patches and you can use a heavier charge.
Also, if you want an adjustable rear sight, go here and order one. The CVA mountain rifle rear sight is adjustable and will fit the spanish made muzzleloaders. 3/8" dovetail i believe.
www.cainsoutdoor.com