Gordon , its not a forgiving design if faithfully followed , if modified it is awesome the amount of energy storage and efficiency you can achive.
Merriick , the best way i have found to build them is to taper your working limbs as if you were aiming them for 1/2 inch tips . but of course the lever shoulders begin way before you become narrow . try 1 3/4 inch at handle fades to 1 to 1 1/8th at the lever sholders. this will give you a more even bend through your working limbs. red oak in this design whould work up to 45 or 50 lbs if your wanting higher try hickory or backing with hickory. hope this helps , ken