CKKOWL is correct. Do a measured sewing piece of 3 strips ... if that is correct ... you are good to go.
It also helps to know your machine. Can you move your needle to the right or left of center? If so, you can then adjust easily when a patterns says sew a "scant" 14" much easier.
Take notes til you don't need them anymore. Also, if you do move your needle for a project find a way to put a note with that project indicating how and how much you moved the needle ... my machines revert to the center needle position each time they are turned off! That can mess you up, too.