The key is to use the same fat monitor each time you do it. And, do it at the same time of day each time.
For instance, I always step on my Tanita at 9 p.m. at night.
As long as it is accurate to ITSELF, than it doesn't matter how much it is off.
If your BF scale says you are 21% today and 19% tomorrow. It is gonna be close enough for gov't work. Even if you are really 24% today and 23% tomorrow. It is showing you progress and that is the reason behind using it.
Don't use the figures to compare yourself to anyone except yourself and it will always work well.