The actual alloy might depend on the calibre and weight of bullet.
The longer the bullet, the harder it can be, within reason.

In the 1800's most hardening was done with the addition of tin or sometimes mercury, which was an older method than tin.

Bullet hardness in the range of 15 to 16 brinel was likely possible, but I would assume not often obtained as a 10:1 ratio of tin:lead would give around 12 to 14 I assume.

In the America's ratios of 20:1, 30:1 and 40:1 were used in the single shot buffalo rifles and match rifles. These are actually quite soft - running about 6 to 10 brinell, I'd estimate. I have not tested those mixes, but perhaps some BP ctg. forum or assoc. has.

I think instead of working to an exact per-determined alloy, one should get testing and use what works.

Due to differences in bore and groove diameters of rifles, tapered, relatively soft paper patched bullets were used. If a smooth sided slug was cast at a diameter that AFTER patching, came to .001" larger than the bore, that slug would shoot amazingly well.

That is how I cast & patched my 460 to 580gr. bullets 2 wraps of .002 for my .45 3-1/4" - making .450" dameter in my rolling block rifle. It used 122gr. of 1F in Bell brass, grease wad, then card, then bullet. It would shoot between 1 MOA and 1 1/2MOA at 100 meters very consistently. The bullets were quite soft, ab out Brinel 8 or 9.

My Brother's Shiloh Sharps in the same chambering preferred 400gr. paper patched bullets, but patched with ordinary masking tape to come up to .451". They were SO easy to patch and shot 1 1/2", 5 shot groups at 100meters. He used 100gr. 2F, grease wad + 10, 1/10" card wads between the grease disk and bullet. Both rifles shot this well 'dirty' - we did not wipe them out between shots.

Paul Mathews books, "The Paper Jacket" and "Loading Black Powder Rifle Cartridges" are both excellent little books to have and learn from. These is where I learned how to patch bullets so they'd shoot well, and well they did shoot in both our rifles. WE had excellent results immediately after reading implementing "Paul's" methods from "The Paper Jacket".