You have more to learn than you can imagine at this point.
It's the forum for Shiloh, but the knowledge there regarding shooting the Holy Black is as deep as the precision shooting knowledge here. Like any interwebz site there is some chaff to sort.
This is the dedicated cast bullet forum!
castboolits.gunloads.com
This site is the tome for knowledge on casting, and casting is as important as the other stuff when it comes to getting a BPCR to shoot.
For lube, use natural lubes for BP that don't contain petroleum based stuff. Cleaning up a BPCR is actually really easy, all it takes is water most times because the natural lubes make fouling that will clean up with plain water, the petrol stuff will make fouling that you need cleaners to dissolve. My advice is to use SPG lube. Someday you may try something else, or not.
Casting. Hard cast bullets rarely work, they never have for me. I've melted all the ones I had down and use them to alloy in soft lead. Most people have success with about 18:1 to 20:1 if I recall correctly. (that's pretty soft) I've been away from my casting pot for a few of yrs now, (trying hard to change that)
Shooting 405ish bullets you're going to want 60 to 70 gr of 1.5F or 2F powder. In the last few yrs I've seen some mention of guys trying 3F for long range stuff. Stick with 1.5F to 2F. Swiss, Schutzen, Goex are all good.
Minimum brass sizing, don't squish the lead bullet. Long range guys don't even size, they slip fit on top of the wad.
Grease Groove Bullets (GG) should be about .001 to .002 over groove size. In modern firearms you can almost bet that a .459 will work, and a .460 will for sure, but not guaranteed. Some of the Italian barrels have been reported to be not exactly .458 every time.
BP burns well with some compression. Usually seen answers that say compress with a wad in the range of .050 to .200 depending on all the variables in your setup.
If you get into Paper Patching (PP) that's a whole 'nuther world.
Edit: and if you want to see just how far you can push a modern 45-70 get Paul Mathews book(s) starting with "Forty Years with the .45/70"