It gives only a rough approximation when you are using pure lead and alloying it with a lead/antimony alloy. Commonly done when you want to change the hardness or run out of a particular batch of lead.
While it is not perfect, the SG of Antimony is about .58. Assuming only two alloying metals (not likely to be exact..) is not pure science, but it does give a working approximation.
art
