I feel the key to natural fiber strings (or any string for that matter) is consistency of strands (diameter, how tight the twist is, etc). Once you are making quality, consistent string, then you can start to collect the data on breaking strength like bjrogg mentions.
I've done a few natural fiber strings. For my dogbane ones I twisted up some different diameter ones that I put to the test with a nail, the vise, and a scale and broke several to see how consistent my results were and to know how many of different sized strands were needed. I'll have to dig up my notes, but the chosen strand size was breaking at some where in the order of 30lbs, so I then knew how many for the bow weight that was needed.
Then I've also done sinew without all the data collection and just twisted up a string that seemed about the right size for a string, and that's worked too. But I still try to keep it very consistent along its length.
I'm sure others with a lot more experience will chime in too. But just mess around with it learn as you go