Don't bother....
I checked into them.
They are "ballpark" accruate for ordinary people. The better ones have an athletic mode to compensate for people with denser muscles, but it still gets thrown off by a bodybuilder.
Might as well use calipers for keeping an idea of if your BF goes up or down and try to get hydrostatically weighed once in a while. If they difference between caliper and hydrostatic remains a consistent number, you can use calipers and do the math.