You'll be fine with the 12 BHN. I cannot imagine .45 ACP needing anything harder. I've pushed pure wheel weight alloy, which is 10-12 BHN in a .45 Colt at some pretty high velocities with no leading whatsoever.
BHN does factor into the equation, no doubt, but how well it fits your bore is just as, if not more important. If you're bullets are sized even a little too small, you risk leading.
Believe it or not, there are some instances where a harder bullet will lead a bore worse than one that is too soft. One of those situations is what I mentioned above. If your bullet is a couple thousandths smaller than your bore, a harder bullet will lead more than a soft one because it doesn't seal off. Gasses escape around it, causing it to melt in the bore.
Sorry for the tangent, but BHN isn't the only factor to look out for. Assuming a good fit, 10 or 12 is plenty hard for the .45 ACP.
Elmer Keith used bullets at a hardness of 11 BHN to develop the .44 mag.