I think the 1/3 figure is a high estimate regarding waste. Waste will be higher initially due to mistakes, do-overs, etc. but as you gain more experience, waste should decrease. When cutting sheath size pieces out of a single or double shoulder, you'll learn to minimize waste. For example, a scar or flaw in the leather can be the section you fold over for a belt loop and it hides the flaw. Sometimes a brand or other scar can add some character. If you mess up a stitch row, save it and use it as a welt or something later. Save trimmings to use as retention straps, spacer pieces, laces and so on. I save scraps down to about 1 1/4 x 1 3/4" for use on stacked leather handles.