I might suggest that you make a template that is large enough so as to square off into the hinge side of the door jamb and square off to the header so that this template is basically about 2/3 of the width of the door and comes down far enough so that will allow you to locate the proper holes for this knocker. You would be able to make it two-sided so that you had one side for left-handed the other side for right hand.
You might consider drilling through the door with a very small drill bits a 1/8 of an inch and then you can check your dimensions on the opposite side of the door to make sure that you've drilled the hole straight through the door rather than at an angle.