Ed, you have raised a very interesting question, and as I have not yet built a horn and sinew wood core bow, I cannot offer any specific recommendations based on experience.
Having played around with backings and given some thought to the nature of composite constructions, I would like to bring to the discussion a few questions that might be pertinent to arriving at an answer to your question. Of course, as always, the devil is in the details, and as these considerations may not be complete, any additional design considerations would be welcomed.
The 1/3 - 1/3 - 1/3 layup seems like a good point of departure for comparing successive bows of similar designs, but is does raise the question of whether the sinew and horn need to be applied in equal amounts of thickness. Why not 1/3 - 2/5 -1/5 ?, or whatever ratios the back and belly are best capable of? Does the sinew and horn have the same stiffness and elasticity?, there by only adding thickness and poundage to the bow without shifting the strains in the core?
Presuming that your hickory is both lighter in mass and stiffer than either the horn or the sinew, why would the core for the composite you describe, be best served by tillering to twenty pounds? Is that how thin it needed to be for your design to flex the full range from reflexed to full draw? Why would you not want it to be as thick/strong as possible, as long as it was capable of bending the full range without taking undue set?