The measured quantity is what ends up in the building. The order quantity is larger. The waste factor is the difference between the two.
The right factor depends on the material, the room shapes and the pattern. Tile laid on a diagonal wastes more than tile laid straight.
Sheet goods with a pattern match waste more than plain ones. Small rooms waste more than large open areas.
Use your own job records where you have them. Show waste on its own line so a reviewer can see it. Check the spec for attic stock, which is extra again.