I think all methods of measuring are based on averages, so keep that in mind. I remember reading that for instance, African Americans have actually been read as negative BF% because they tend to have more muscle mass and denser bones. Anything based on averages is only going to be as good as the data that goes into it, and how close to the median you are. As long as you're trending down, you're on the right track.
Here's another calculator and a chart to illustrate what I mean:
Home Body Fat Test