The ornamental design for a body fat scale, as shown and described.
FIG. 1 is a plan view of the body fat scale embodying my design;
FIG. 2 is a right side view thereof, the left side view being a mirror image of the right side view;
FIG. 3 is a shaded plan view of a variation of my design; and,
FIG. 4 is a shaded right side view thereof, the left side view being a mirror image of the right side view.
It will be understood that FIGS. 1 and 3 show the body fat scale with digits within an LED readout, text disposed above the readout and three vertically spaced buttons which have text and/or graphics therein, these features being shown for environmental purposes and forming no part of the claimed design.