re: body fat scales - most are not that accurate. Their value lies in their being consistently inaccurate. So, they are good as a tracking tool, but you shouldn't necessarily take them too seriously.

The problem with the bathroom scale version is that is measures via impedence, so a current travels up one leg, across the pelvis and down the other leg. Leaving out half your body.

If you get your body fat measured by a trained professional with calipers, and find out that it is 5% or 7% different than your scale reading (yes, they are usually that far off!), you can use that figure to adjust all your readings. It's also my understanding that the ones to buy are the kind that also measure your hydration, because how hydrated you are makes a big difference in your reading.

I have a bf scale, but I really only use it to give myself a boost (I enjoy watching the % drop)... for someone who really cares about bf and needs to know exactly - you should have it done by a professional, as well, and then you can use a home scale to keep track.