My mix is to mimic Lyman #2 Alloy, about BHN 15, which is what a lot of the Lyman mold diameters are cut for. If you use a different alloy they may come out heavier or lighter and a little narrower or a little wider. The emphasis here is on 'little'. All well within a normal sizing die's ability to correct.
2% tin is used in Terracorp Magnum Alloy (and is usually cited as the minimum amount of tin needed to improve mold fill and castability) which also has 6% antimony, IIRC. It's another good casting alloy if you want about BHN 16 as-cast. But if you use 50:50 WW plus 2% tin, you'll have about 2% antimony and 2% tin, which won't be as hard as cast, being about BHN 10-12 or so (again IIRC). But because of small amounts of arsenic in the WW it will still be possible to harden them by water quenching up to about BHN 25 or so, if you want to, and without getting brittle, and because the tin content does not exceed the antimony content, it should settle at about BHN 21-22 and stay there for a decade or more.
Theres a
book on line at the Los Angeles Silhouette Club site that you might want to read.