The old way of determining your draw length, particularly with a straight grip, dish, or locator on a longbow is to 1) use a wooden yard stick, put it at the top of your chest (centered), below collar bone and reach out with both arms, do not strain, and have someone mark where your finger tips are with a pencil, and don't look. 2) repeat this two more times, and then average the three lengths, to get your correct draw length. This will get you closer to the best draw length for your size.
If you develop proper form, heeling the bow, relaxed arm, low bowarm shoulder, you probably will have better success. The biggest problem is thinking a longer draw is better; with a longbow it isn't. The correct draw length for a 68" traditional longbow is 28". You subtract or add 2" to the bow length, for each inch under or over 28".
This general rule has been used for 100 years,it worked for Art Young, Dr. Saxton Pope, Howard Hill and it will work for you.