There are a few sources on the net too. Try searching various terms including "cordage" and "cable". Both here and Paleoplant have some examples. This is one of the best sources: http://www.primitiveways.com/cordage_backed_bow.html
But to answer your question of how much, there really is no firm answer. I have see numbers about how much sinew is required, but that was to replicate one bow in particular. One thing about cordage backed bows is that they can be make with various forms of backing, depending on where and how many knots and how many strings, so the amount needed will depend on style.
I have read that silk would not be good for cordage backing because it does not ... what is the word, rebound?... much after stetching. Of course sinew is the best. Nylon is second best. Perhaps poly and things like nylon would be better than other natural fibers. Of the natural fibers, Tim Baker suggested sisal as the next best in elasticity.
If I were to use silk, I would use more than I though was necessary and expect to constantly tighten it up. That way I thing you could slowly use of the "give" in the silk.
