Be aware of an experience I had several years ago (with Delta Air Lines) in Tampa, that 3 scales adjacent to each other displayed 3 different weights. The one used at check-in showed over, the other 2 under. After calling a supervisor, and proving the point, no more was said and the suitcase was duly accepted at no extra charge. So, the gadget above (depending of course on its own accuracy), can be a reasonably good guideline. We have something similar in our home (only for my wife's suitcase)!!!
