I have found this to be the most accurate system for finding your true draw length with a long bow when anchoring to the corner of your mouth.
Take a tape measure or yard stick, and place the butt end of the tape against your sternum. Reach straight forward with both hands and sandwich the yard stick between your hands. Now relax your shoulders, and check the measurement indicated just at the tip of your middle finger.
The measurement may be shorter than you expect but this will indicate your true draw length with a longbow.
If you have broad shoulders you may actually have a longer draw length than indicated.