I've tried the hemp cordage, and wasn't too enamored with it. The sinew will make a better string, but will be more work. The breaking strength of your string should be about 4 times the draw weight of the bow. If you're making a string from several strands, measure one strand and use however many you need to get the total breaking strength you need. Sinew strings are usually made two-ply, though, so you'll have to make a short piece and test it to gauge the final thickness.
