Ditto! You probably won't be off that much once you get the arm and the protractor in the correct positions. It sounds like most likely you haven't learned how to do this important step with an arc-style protractor. As Swampwalker and Thom say, you need to get the arc tracing correct first.
To do that you will need to move both the protractor and arm until your stylus is hitting the arc at the extreme outside edge and as close to the spindle as possible. Once you have that done you will have found the place to lock down the protractor. It is all in the directions. Read them again, be sure you understand, take your time.