I can swing a handheld by 5-10% in one day by increasing sodium and water intake. It measure the impedance (speed the electricity travels through your body) Increasing your state of hydration and electrolytes will totally fool the that hand held by increasing the speed the electricty travels through your body. The faster it travels the lower it thinks your bodyfat is since muscle is 80% water it assumes your LBM is much higher. If you are lean and dehydrated it will read a much higher bodyfat level due to slower electric conduction, and on the flip side if you are a little soft but well hydrated it can read much lower than you are. It is probably as useless of a device as the old school BMI charts since it relies on such an easily modified method of measurement.
You would probably be better off just taking progress pics, and or getting some calipers just to make sure the pinch measurements are moving down. The actual formula is not needed so long as you see the pinch measurement getting lower your bodyfat is too.
You look like you are kicking some ass and should be happy with your progress.