Great question. Ditto on much of what was said here. I've learned some of it the hard way!
I have two 1858s made by Pietta, and a couple Navy revolvers (also made by Pietta). I use Pyrodex pellets (because I've become too lazy to measure real black powder!), which are 30 gr. equivalent to FFFg. I place a wad on top of the pellets and a .454 ball on top of that. I then place a liberal amount of Bore Butter on each cylinder. I actually experienced a chain fire once using wads with no Bore Butter. Kind of scary. Didn't hurt anyone or my gun (much), but scared the bejeezus out of me enough to be a tad over cautious from then on...
These things are fun to shoot! I really can't speak to your accuracy problem except to agree with others that your powder load is perhaps a bit low. And herein lies what I found odd from the start: The Pietta manual (on p. 10, if they are still printing the same one I've had for years) says the cylinder capacity is 12-15 gr. of FFFg and recommends a .454" round ball. BUT the Cabela's manual, which also came with at least one of my pistols, sets the cylinder capacity at 35 gr. of FFFg black powder OR 28 gr. of Pyrodex with a .451" round ball. So go figure, since that's a decent amount of difference between the two manuals... I've shot my 1858s with the "max" listed by Cabela's with no problem, and the accuracy is not bad (although I couldn't give you a measurement off the top of my head, as it's been a while since I shot it at paper that I could measure).
Best of luck to ya! Hope you remedy your accuracy issue.