I've had problems measuring body fat. Take 2 different machines, get 2 different answers. Use an online calculator, get a different answer again. My % ranges from 18 - 28%.
Trouble is, there isn't a totally reliable method short of autopsy, which I presume isn't on your agenda!
Therefore, pick a method and stick to it. I've more or less given up on the idea and will do the old fashioned methods of scales, mirror and trousers.
There is one method I've been told about, however, that does seem to get the most accurate results:- Hydrostatic weighing. It involves weighing you dry and then weighing you totally submerged in water. Don't ask me how it works, but apparently it does. Have a Google and see if it exists in your locale. Personally, I'm not going to bother with it.