I am having a problem making simple sketches.  Often the initial starting line shifts when I add additional dimensional lines to the sketch.  I tried minimizing this problem when I want to create a rectangular shape by choosing a "Center Rectangle" hoping that it will retain it's shape when I add the dimensions to the shape.  I have added a photo showing what happens when I did this.  Notice that it isn't just the shape that is distorted, but also the scale is different between the vertical and horizontal lines.   The horizontal line may be hard to read, it is 18" long.   Is there some secret that I am missing?

Thanks