Let's calibrate your tape measure. Do you have a combination square? No? Then time to get one. And spend a little money on one with very crisp 1/64" markings. Next, locate a nice crisp edge item that is flat. A metal table top or finished piece of hardwood trim like oak. Set your combination square for a nice even number like 6 inches. Use a very sharp pencil and mark the flat surface with the square held firmly on the edge. Next, hook your tape measure over the edge, pulling tight, your tape should read precisely 6". Next, remove the rigid ruler from your combination square. You will use the right angle part next. Push the vertical side against the crisp edge. Push the end of your tape measure against the vertical side of the combination square. It should read again exactly 6". If it doesn't, then your tape measure end is damaged or just such poor quality that the scaled lines are inaccurate.