[joshvm]

I don't trust the absolute numbers, but the data from my FitBit Aria has a pretty low variance over time:

http://imgur.com/5AgKU9F

It does seem to be tracking reasonably well - I've lost a bit of fat, but not a significant amount since then. Given what I see in the mirror, I think 16-17% is about right. There are some weird fluctuations, but we're talking 1-2% day to day. The range of the graph is about 4-5%.

The trick is to measure several times a day if you get the chance (or at the very least when you get up and when you go to bed) to smooth out any fluctuations.