Found the following. Should apply to an AR.
The length of pull of a rifle is often confused with a shotgun length of pull. Two totally different things. Factory rifles come with a 13-1/2 length of pull. On a rifle your eye has to be in a relative position to see thru the scope. In a shotgun the feel of the gun and the relationship of your finger to the trigger are important, as is your ability to quick point an object and be looking down the barrel at it every time. A measurement we use is to bend your elbow 90 degrees. Place a yardstick on the inside of your arm in the crotch and measure from there to the center of your 90 degree trigger finger. This works pretty well, and that measurement is your length of pull. Another way is to measure a rifle you have that fits you. To do this measure from the back edge of the butt pad to the trigger. That would give us a measurement we could duplicate on your new Jarrett. The rifle will be delivered with the length of pull you want. A length of pull mistake on the customer's part can be corrected but it's not warranty work so be careful. Rule of thumb, for a person 5' 8" tall to 6' 2" tall a 13-1/2" length of pull is perfect.