What about if you measure yourself at the same exact time each day you do it (say immediately after you wake up)?
Still a poor idea to use the scale because your sodium levels fluctuate from day to day. If you ate the exact same food and did the exact same thing every day and used bioelectric impedance.... it would work in theory.... but still not in practice.
The reason being that most of the scales don't just use bioelectric impedance. Most ask you for your height and weight and use a formula from your height, weight, and a bioelectric impedance measure to infer your bodyfat. You can test it by measuring your bodyfat, grabbing a 20 pound plate and remeasuring your bodyfat. It's won't go down like it should!
Honestly, if you are already pretty lean then it's pretty hard to measure your bf accurately. Your waist measurement and a mirror are a pretty good gauge of how lean you are for most people.