Andy,
I own a Tanita and use it regularly. I experience wide fluctuations from morning to mid-day. I believe this is due to water. For me, in the morning my
bodyfat percentage is higher and bodyweight lower, whereas in the afternoon, it's just the opposite.
For example, I might weigh 180 lbs. and 16.5% bodyfat in the morning, and by mid-day I might weigh 182 lbs. and 11% bodyfat.
So, I weigh myself each morning at the same time, and that's what I use. I believe that my bodyfat is lower than the morning measurement but that's what I use.
The value of this is how the readings change over time.
I personally like the mirror test. Can I see my cheek bones, and can I see my hip bones. Can I see some ab definition. Generally when the answer to all of these is yes, then my bodyfat percentage is where I want it to be.