Hi Shay.
As a general rule of thumb.
If you place Brendons arms at his sides, then bend his shooting arm out in front of him at 90 degrees, get him to form a pistol grip with his hand ( naturaly ) now with a tape measure, from the cleft of the arm, to the finger pad ( Trigger Finger ), measure that distance.
That should give you a very good guide to his natural length of pull, make allowances for shooting clothing too.
Once you have the length of pull you can work on combe length hieght etc.
Hope you can understand that dribble
My length of pull is 15 inches