Welcome to the forum, Cory.
If the cleaning rods you use for you centerfire guns are long enough, and have the rotating handles, they are the best bet for checking your barrel's rate of twist. Use the largest slotted patch holder that will fit, along with a large patch. A 2" square might work, but make sure it fits tightly enough to force the rod to turn with the rifling. Use a magic marker to put a witness mark on the rod and watch it twist while you keep the handle from turning.
The conventional wisdom says that slow twist barrels (1 in 60" or so) are best suited for patched round balls, while faster twists (1 in 32" or so) are better for conical bullets. A twist of 1 in 48" is considered the compromise between the two.
As you know, balls and patches come in different sizes. If using a 0.570" ball, you probably want a patch of 0.015" - 0.018" thickness. If you go with a 0.575" ball, go down to 0.010" in patch thickness. Vary to suit your particular bore... you don't want the fit to be so tight that you risk snapping the ramrod and impaling the palm of your hand while trying to load.
A good web-source for balls, conicals, and patches is
Track of the Wolf, but you can find other suppliers listed on the "Links" page of this forum.
Commercial lubes are fine, but I've had the best luck with stuff I make using a recipe concocted by our very own Stumpkiller.
Stumpkiller's Moose Milk
Isopropyl Alcohol (91%) __ 8 oz.
Castor Oil _______________ 3 oz.
Murphy's Oil Soap ________ 1 oz.
Witch Hazel ______________ 4 oz.
Water (non-chlorinated) __ 16 oz.
Add in the above order and shake well to mix as added, and again before every use or transfer. Oil and water don't mix, dontcha know. Castor is alcohol soluable, but not in water. And yes, the witch hazel does seem to make a difference in how well it smears and coats the bore.
For powder, I prefer GOEX... real black powder rather than a substitute. The conventional wisdom says use 2F for calibers above 50, and 3F for calibers below that. Opinions vary, of course... some folks use 2F in everything, some use 3F in everything. I use 3F in my .54 cal.
The gun's maker is the best source for a max powder charge, in the expectation that he has done some sort of proofing. In the absence of that, you might turn to Lyman's BP Handook (available from Lyman's web site or
[url] Amazon.com[/url]). My copy shows GOEX 2F charges for that caliber of patched round ball as low as 50 grains and as high as 190 grains. I usually start with a powder charge of grains equal to 1.5 times the caliber and work up or down from there. In the case of a .58 cal, I would start with 90 grains of 2F or 3F and move up by 5 grain increments until I found the best balance of power and accuracy. The handbook gives a MV of 1149 fps from a load of 90 grains of GOEX 2F. Folks that have been shooting .58s might have some ideas about how much higher than that to go to get a good hunting load.
As for references, you might take a look at
Lyman's BP Handbook, and
Bob Spencer's web-based BP Notebook has lots of useful information.