I don't recommend wasting your money on a Tanita or other BIA (Bioelectric Impedance Analyzer) instrument - they REALLY lie. Body fat measurement by BIA is a valid method - IF LOTS of things are done properly and if LOTS of other factors are taken into account including the room temperature and humity and your height and your heritage.

Cheap (relative to the many thousands of dollars a proper set up would cost) BIA scales are notoriously inaccurate and, worse, imprecise. You are primarily measuring your skin resistance which varies all over the place. Far better tracking of changes in body fat percentage can be done by taking your key measurements with a cloth tape. The body fat percentage that results from using those measurements and any of the various formulas around is of questionable accuracy, but if you use the same method over time you will get a pretty good indicator of changes in body fat percentage.