I recently tried 2 BF% measurement scales and both were off in the sense that they both gave me different numbers. One had more variation than the other when measuring during consistent times, etc. So, I returned both and bought $20 calipers from accu-measure (see
http://www.bodybuilding.com/store/acc/accu.html). I now measure my BF% on the basis that I don't actually calculate the number; I just look at the calipers' measure scale (millimeters?). So, if that goes down, great. If not, ouch. This is how I keep track. Once in a while I go through the calculations. My weight is steady/constant, so I don't have to worry about calculating in weight with the measurements. I think that the calipers are much more accurate! I would not waste my money on a scale... Too many people don't like them. If you are so serious about your BF% (which in my opinion should not be such an issue; look at how your clothes fit and at how you look, etc.), then buy calipers or get measured in a water tank once in a while.
Hope this helps!
Anna