It wasn't too awfully long ago that I asked about the same question over on CB's. Here are a couple of those post.
Tin in HP's
Whats your HP alloy
Since then I have played with the large iso core alloy a bit and found a few things out. The biggest was that you should have a balance of tin to antimony if you want them to be pliable. If your using known alloy to begin with it makes it a bit easier than if your starting out with mystery metal since then you can figure what needs to be added to result in what your after.
As far as BHN goes, most of that has been posted in the two post above is pretty soft by most standards running from roughly a 8 to 12 BHN. As mentioned in the one post the particular cavity shape and depth as well as the velocity will also come into play. The key thing is to have a pliable alloy that will allow the nose to roll back without fracturing off. To get this one can only test what your pouring and go from there.
Like I did with my HP alloy in the post below, it works great for the ACP and the Colt, but fired in the 41 or 44, it will result in a lost ring every shot.
Pouring HP's and Learning to Blend Alloy's....(bit long)
Cast - VS - Jacketed
Granted I started out with more or less a known base alloy and blended it with other known alloys to come to the results I got. I have since tried to stiffen this up a bit to use in the 41 and 44 with similar type bullets, but haven't gotten it where I want it yet. I just haven't had time to reconfigure what I have to see where it needs to go. As you know sometimes life in general gets in the way of what we would rather be doing.
So anyway there ya have it. I hope that helps some. I know how much I put into getting to where I am with the one alloy, when it might have been just as simple to start with pure lead and go from there. Trouble is I don't have a big supply of pure.