I find it helpful in figuring out how to mix an alloy with the percentages I am after. I do not look at the expected BHn as I can move that up by heat treating if the alloy contains Sb.
if that's Bumpo's calculator it isn't perfect, and doesn't work well until you get things over 8-9 bhn.
it can show soft/purish lead as 8 so make sure you pay attention to that.