I have run into a problem, and maybe it's a simple solution but for the life of me I can't figure it out. I am making a wallet and when I measure and cut, the angles of the corners keep getting off. The sides are parallel, but they are not perpendicular. How do you get the sides parallel, but also to be perpendicular? I have thought to use a square ruler, but the slightest movement when making the second cut will cause the angle to shift. This means that when I lay the parts of the wallet together I get overlap along the edge. I included an exaggerated picture to better describe the problem.